Filed pursuant to Rule 424(b)(4)
Registration No. 333-225412

Prospectus

5,600,000 Shares

Class A common stock

$35.00 per share

We are offering 3,292,580 shares of our Class A common stock and the selling stockholders identified in this prospectus are offering 2,307,420 shares of our Class A common stock. We will not receive any of the proceeds from the shares of Class A common stock sold by the selling stockholders.

Our Class A common stock is listed on the NASDAQ Global Select Market under the symbol “ALTR.” On June 6, 2018, the last reported sale price of our Class A common stock as reported on the NASDAQ Global Select Market was $36.29 per share.

We have two classes of common stock, Class A common stock and Class B common stock. The rights of the holders of Class A common stock and Class B common stock are identical, except with respect to voting and conversion rights. Each share of Class A common stock is entitled to one vote per share. Each share of Class B common stock is entitled to ten votes per share and is convertible into one share of Class A common stock. The holders of our outstanding Class B common stock will hold approximately 91% of the voting power of our outstanding capital stock following this offering, with our directors, executive officers and current beneficial owners of 5% or more and their affiliates holding approximately 91% of the voting power.

We are an “emerging growth company” as defined under federal securities laws. We anticipate that following the completion of this offering, our chairman and chief executive officer will continue to control a majority of the combined voting power of our common stock. While under NASDAQ rules we may be deemed a “controlled company,” we do not intend to take advantage of the corporate governance exemptions afforded to a “controlled company” under NASDAQ rules.

Investing in our Class A common stock involves a high degree of risk. Please see the section titled “Risk factors” starting on page 17 to read about risks you should consider carefully before buying shares of our Class A common stock.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or passed upon the adequacy or accuracy of this prospectus. Any representation to the contrary is a criminal offense.

We have granted the underwriters a 30-day option to purchase up to an additional 840,000 shares of Class A common stock at the public offering price, less the underwriting discount.

The underwriters expect to deliver the shares on or about June 11, 2018.

The date of this prospectus is June 6, 2018

Table of contents

Neither we, the selling stockholders, nor the underwriters have authorized anyone to provide any information or to make any representations other than those contained or incorporated by reference in this prospectus or in any free writing prospectuses we have prepared. We and the selling stockholders take no responsibility for, and can provide no assurance as to the reliability of, any other information that others may give you. We and the selling stockholders are offering to sell, and seeking offers to buy, shares of our common stock only in jurisdictions where offers and sales are permitted. The information contained or incorporated by reference in this prospectus is accurate only as of the date of this prospectus or the date of the applicable document incorporated by reference, regardless of the time of delivery of this prospectus or any sale of shares of our common stock. Our business, financial condition, results of operations, and prospects may have changed since that date.

For investors outside the United States: Neither we, the selling stockholders, nor the underwriters have done anything that would permit this offering or possession or distribution of this prospectus in any jurisdiction where action for that purpose is required, other than in the United States. Persons outside the United States who come into possession of this prospectus must inform themselves about, and observe any restrictions relating to, the offering of the shares of our common stock and the distribution of this prospectus outside of the United States.

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Risk factors

Investing in our Class A common stock involves a high degree of risk. You should carefully consider the risks and uncertainties described below, as well as those set forth under the heading “Risk Factors” in our Annual Report on Form 10-K for the year ended December 31, 2017, as updated by our Quarterly Report on Form 10-Q for the quarter ended March 31, 2018, which are incorporated by reference in this prospectus. Our business, operating results, financial condition or prospects could be materially and adversely affected by any of these risks and uncertainties. In that case, the trading price of our Class A common stock could decline and you might lose all or part of your investment. In addition, the risks and uncertainties discussed below and in the documents incorporated by reference are not the only ones we face. Our business, operating results, financial condition or prospects could also be harmed by risks and uncertainties not currently known to us or that we currently do not believe are material, and these risks and uncertainties could result in a complete loss of your investment. In assessing the risks and uncertainties described below, you should also refer to the other information contained in this prospectus (as supplemented or amended) and the documents incorporated by reference in this prospectus, including our consolidated financial statements and the related notes thereto, before making a decision to invest in our Class A common stock.

Risks related to this offering and ownership of our Class A common stock

Our management has broad discretion in the use of the net proceeds from this offering, and our use of the net proceeds may not enhance our operating results or the price of our Class A common stock.

We intend to use the net proceeds to us from this offering for general corporate purposes, including working capital, sales and marketing activities, general and administrative matters, real estate development, including developing and building an addition adjacent to our corporate headquarters facilities, and capital expenditures. We may also use a portion of the net proceeds for the acquisition of, or investment in, technologies, solutions or businesses that complement our business. While we do not have agreements or binding commitments for any specific acquisitions at this time, we continually evaluate potential acquisition candidates to enhance our product offerings. Accordingly, our management will have considerable discretion over the specific use of the net proceeds that we receive in this offering and might not be able to obtain a significant return, if any, on investment of these net proceeds. You will not have the opportunity as part of your investment decision to assess whether the net proceeds are being used appropriately. Investors in this offering will need to rely upon the judgment of our management with respect to the use of proceeds. Until the net proceeds are used, they may be placed in investments that do not produce significant income, may be held in demand deposit accounts, or in investments intended to be highly liquid that may nevertheless lose value. If we do not use the net proceeds that we receive in this offering effectively, our business and prospects could be harmed, and the market price of our Class A common stock could decline.

As a new investor, you will experience immediate and substantial dilution in the book value of the shares that you purchase in this offering.

The public offering price is substantially higher than the net tangible book value per share of our Class A common stock immediately following this offering based on the total value of our tangible assets less our total liabilities. Therefore, if you purchase shares of our Class A common stock in this offering, at the public offering price of $35.00 per share, you will experience immediate dilution of $33.65 per share, the difference between the price per share you pay for our Class A common stock and our net tangible book value per share as of March 31, 2018, after giving effect to the issuance of shares of our common stock in this offering. See the section titled “Dilution” for more information.

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Our Class A common stock has only recently become publicly traded, and the market price of our Class A common stock may be volatile.

The market price of our Class A common stock may fluctuate substantially depending on a number of factors, many of which are beyond our control and may not be related to our operating performance. These fluctuations could cause you to lose all or part of your investment in our Class A common stock, since you might not be able to sell your shares at or above the price you paid for our Class A common stock. Factors that could cause fluctuations in the market price of our Class A common stock include the following:

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In addition, the stock market in general, and the market for technology companies in particular, have experienced extreme price and volume fluctuations that have often been unrelated or disproportionate to the operating performance of those companies. Broad market and industry factors may affect the market price of our Class A common stock, regardless of our actual operating performance. In the past, following periods of volatility in the overall market and the market prices of a particular company’s securities, securities class action litigation has often been instituted against that company. We may become the target of this type of litigation in the future. Securities litigation, if instituted against us, could result in substantial costs and divert our management’s attention and resources from our business.

Sales of substantial amounts of our Class A common stock could cause the market price of our common stock to decline and may dilute your voting power and your ownership interest in us.

Sales of a substantial number of shares of our Class A common stock after this offering, particularly sales by our directors, executive officers and significant stockholders could adversely affect the market price of our Class A common stock and may make it more difficult to sell Class A common stock at a time and price that you deem appropriate. Based on the total number of outstanding shares of our common stock as of March 31, 2018, upon completion of this offering, we will have an aggregate of 32,582,012 shares of Class A common stock and 34,832,256 shares of Class B common stock outstanding, assuming no exercise of our outstanding stock options or settlement of restricted stock units after March 31, 2018 and assuming the underwriters do not exercise their option to purchase additional shares.

All of the shares of Class A common stock sold in this offering will be freely tradable without restrictions or further registration under the Securities Act of 1933, as amended, or the Securities Act, except for any shares held by our affiliates as defined in Rule 144 under the Securities Act. Shares held by directors, executive officers and other affiliates will be subject to volume limitations under Rule 144 under the Securities Act and various vesting agreements.

In addition, as of March 31, 2018, there were 8,877,036 shares of Class A common stock subject to outstanding options and 58,295 shares of Class A common stock to be issued upon the vesting and settlement of outstanding RSUs. We have registered an aggregate of 17,845,247 shares of Class A common stock issuable (i) upon exercise of outstanding options, (ii) upon the vesting and settlement of outstanding RSUs and (iii) upon exercise or settlement of any options or other equity incentives we may grant in the future for public resale under the Securities Act. Accordingly, these shares may be freely sold in the public market upon issuance as permitted by any applicable vesting requirements, subject to the lock-up agreements described below and compliance with applicable securities laws.

In connection with this offering, we, the selling stockholders and our directors and executive officers have agreed with J.P. Morgan Securities LLC and RBC Capital Markets, LLC on behalf of the underwriters in this offering, that for 90 days after the date of this prospectus (in the case of us and the selling stockholders) or 60 days after the date of this prospectus (in the case of our directors and executive officers who are not selling stockholders), subject to certain exceptions, we and they will not, directly or indirectly, dispose of any of our common stock or securities convertible into or exercisable or exchangeable for our common stock. See the section entitled “Shares eligible for future sale” for more information.

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Sales of a substantial number of such shares upon expiration of the lock-up agreements, or the perception that such sales may occur, or early release of these agreements, could cause our market price to fall or make it more difficult for you to sell your Class A common stock at a time and price that you deem appropriate.

We may also issue shares of our Class A common stock or securities convertible into shares of our Class A common stock from time to time in connection with a financing, acquisition, investment, our employee incentive compensation plan or otherwise. Any such issuance could result in substantial dilution to our existing stockholders and cause the market price of our Class A common stock to decline.

We are a “controlled company” within the meaning of the rules of the NASDAQ Stock Market and, as a result, we qualify for, but do not currently intend to rely on, exemptions from certain corporate governance requirements. In the event that we were to rely on these exemptions in the future, you would not have the same protections as are afforded to stockholders of companies that are subject to such requirements.

Upon completion of this offering, James R. Scapa, our chairman and chief executive officer, will continue to control a majority of the combined voting power of our company. As a result, we will continue to be a “controlled company” within the meaning of the NASDAQ corporate governance standards. Under the NASDAQ rules, a company of which more than 50% of the voting power is held by an individual, group or another company is a “controlled company” and need not comply with certain requirements, including the requirement that a majority of the Board consist of independent directors and the requirements that our compensation and nominating and governance committees be composed entirely of independent directors. Since our initial public offering, we have not taken advantage of, and following this offering we do not intend to take advantage of, these exemptions. However, for so long as we qualify as a “controlled company,” we have the authority to utilize some or all of these exemptions. If we utilize these exemptions, among other things, we may not have a majority of independent directors and our compensation and nominating and governance committees may not consist entirely of independent directors. Thus, in the event that we were to rely on these exemptions in the future, you would not have the same protections as are afforded to stockholders of companies that are subject to all of the corporate governance requirements of NASDAQ.

The dual class structure of our common stock has the effect of concentrating voting control with certain stockholders who hold shares of our Class B common stock, including our founders, certain of our directors and executive officers and affiliates, who will hold in the aggregate 91% of the voting power of our capital stock following the completion of this offering. This will limit or preclude your ability to influence corporate matters, including the election of directors, amendments of our organizational documents, and any merger, consolidation, sale of all or substantially all of our assets, or other major corporate transaction requiring stockholder approval.

Our Class B common stock has ten votes per share, and our Class A common stock, which is the common stock we are offering pursuant to this prospectus, has one vote per share. Following this offering, our Class B stockholders, including our founders, certain of our directors and executive officers, and affiliates, will hold, in the aggregate 91% of the voting power of our capital stock. The ten-to-one voting ratio between our Class B and Class A common stock, results in the holders of our Class B common stock collectively controlling a majority of the combined voting power of our common stock and therefore being able to control all matters submitted to our stockholders for approval until 2029, or upon the occurrence of a triggering event at which time all shares of our Class B common stock will automatically convert into shares of our Class A common stock, or on an earlier date, as set forth in our certificate of incorporation.

This concentrated control will limit or preclude your ability to influence corporate matters for the foreseeable future, including the election of directors, amendments of our organizational documents, and any merger, consolidation, sale of all or substantially all of our assets, or other major corporate transaction requiring

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stockholder approval. In addition, this may prevent or discourage unsolicited acquisition proposals or offers for our capital stock that you may feel are in your best interest as one of our stockholders.

Future transfers by holders of our Class B common stock will generally result in those shares converting to Class A common stock, subject to the specific exceptions set forth in our certificate of incorporation, such as certain transfers effected for estate planning purposes and between or among our founders. The conversion of Class B common stock to Class A common stock will have the effect, over time, of increasing the relative voting power of those holders of Class B common stock who retain their shares in the long-term. For a description of the dual class structure, see the section entitled “Description of capital stock—Anti-takeover effects of Delaware law and our certificate of incorporation and bylaws.”

Our bylaws designate courts in the State of Delaware as the sole and exclusive forums for certain types of actions and proceedings that may be initiated by our stockholders, which could limit our stockholders’ ability to obtain a favorable judicial forum for disputes with us or our directors, officers, or employees.

Our bylaws provide that, subject to limited exceptions, a state or federal court located within the State of Delaware will be the sole and exclusive forums for:

Any person or entity purchasing or otherwise acquiring any interest in shares of our capital stock are deemed to have notice of and to have consented to these provisions. These provisions may limit a stockholder’s ability to bring a claim in a judicial forum that it finds favorable for disputes with us or our directors, officers, or other employees, which may discourage such lawsuits against us and our directors, officers, and employees. Alternatively, if a court were to find these provisions of our bylaws inapplicable to, or unenforceable in respect of, one or more of the specified types of actions or proceedings, we may incur additional costs associated with resolving such matters in other jurisdictions, which could adversely affect our business and financial condition.

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Special note regarding forward looking statements

This prospectus and the documents incorporated by reference in this prospectus, particularly in the sections titled “Prospectus Summary” and “Risk Factors” in this prospectus and the sections titled “Risk Factors,” “Management’s Discussion and Analysis of Financial Condition and Results of Operations” and “Business” in our Annual Report on Form 10-K for the year ended December 31, 2017 and our Quarterly Report on Form 10-Q for the quarter ended March 31, 2018, include forward-looking statements within the meaning of Section 27A of the Securities Act of 1933, as amended, and Section 21E of the Securities and Exchange Act of 1934, as amended. All statements other than statements of historical facts contained in this prospectus, including statements regarding our future results of operations and financial position, strategy and plans, market size and opportunity, competitive position, industry environment, potential growth opportunities and our expectations for future operations, are forward-looking-statements. The words “believe,” “may,” “will,” “estimate,” “continue,” “anticipate,” “design,” “intend,” “expect,” “could,” “plan,” “potential,” “predict,” “seek,” “should,” “would” or the negative version of these words and similar expressions are intended to identify forward-looking statements. We have based these forward-looking statements on our current expectations and projections about future events and trends that we believe may affect our financial condition, results of operations, strategy, short- and long-term business operations and objectives, and financial needs. These forward-looking statements are subject to a number of risks, uncertainties and assumptions described under the section titled “Risk Factors” in this prospectus and in our Annual Report on Form 10-K for the year ended December 31, 2017, as updated by our Quarterly Report on Form 10-Q for the quarter ended March 31, 2018, which are incorporated by reference in this prospectus, and elsewhere in this prospectus and in the documents incorporated by reference in this prospectus, regarding, among other things:

These risks are not exhaustive. Other sections of this prospectus and the documents incorporated by reference in this prospectus may include additional factors that could adversely impact our business and financial performance. New risk factors emerge from time to time and it is not possible for our management to predict

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all risk factors, nor can we assess the impact of all factors on our business or the extent to which any factor, or combination of factors, may cause actual results to differ materially from those contained in, or implied by, any forward-looking statements.

You should not rely upon forward-looking statements as predictions of future events. Although we believe that the expectations reflected in the forward-looking statements are reasonable, we cannot guarantee future results, levels of activity, performance or achievements. Except as required by law, we undertake no obligation to publicly update any forward-looking statements for any reason.

You should read this prospectus, the documents incorporated by reference in this prospectus and the documents that we have filed as exhibits to the registration statement on Form S-1, of which this prospectus is a part, that we have filed with the SEC with the understanding that our actual future results, levels of activity, performance and achievements may be materially different from what we expect.

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Market, industry and other data

We obtained the industry, market and competitive position data used throughout this prospectus from our own internal estimates and research, as well as from industry and general publications, in addition to research, surveys and studies conducted by third parties. Internal estimates are derived from publicly-available information released by industry analysts and third party sources, our internal research and our industry experience, and are based on assumptions made by us based on such data and our knowledge of our industry and market, which we believe to be reasonable. In addition, while we believe the industry, market and competitive position data included in this prospectus is reliable and is based on reasonable assumptions, such data involves risks and uncertainties and are subject to change based on various factors, including those discussed in the section entitled “Risk factors” in this prospectus and in the documents incorporated by reference in this prospectus. These and other factors could cause results to differ materially from those expressed in the estimates made by the independent parties and by us.

Information based on estimates, forecasts, projections, market research, or similar methodologies is inherently subject to uncertainties, and actual events or circumstances may differ materially from events and circumstances that are assumed in this information. In some cases, we do not expressly refer to the sources from which data is derived.

Certain information in this prospectus or in documents incorporated by reference in this prospectus is contained in independent industry publications. The source of these independent industry publications is provided below:

The independent publications described herein or in the documents incorporated by reference in this prospectus represent research opinions or viewpoints published and are not representations of fact. Each publication speaks as of its original publication date (and not as of the date of this prospectus) and are subject to change without notice.

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Use of proceeds

We estimate that the net proceeds from our sale of 3,292,580 shares of Class A common stock in this offering at the public offering price of $35.00 per share, after deducting underwriting discounts and commissions and estimated offering expenses payable by us, will be approximately $109.3 million, or $137.3 million if the underwriters’ option to purchase additional shares is exercised in full.

We intend to use the net proceeds to us from this offering for general corporate purposes, including working capital, sales and marketing activities, general and administrative matters, real estate development, including developing and building an addition adjacent to our corporate headquarters facilities, and capital expenditures. We may also use a portion of the net proceeds for the acquisition of, or investment in, technologies, solutions or businesses that complement our business. We do not have any agreements or binding commitments for any such acquisitions or investments at this time.

We will have broad discretion over the uses of the net proceeds in this offering. Pending these uses, we may invest the net proceeds from this offering in short-term, investment-grade interest-bearing securities such as money market accounts, certificates of deposit, commercial paper and guaranteed obligations of the United States government.

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Dividend policy

We have never declared or paid and do not anticipate declaring or paying, any cash dividends on our Class A common stock. We currently intend to retain all available funds and any future earnings for use in the operation of our business. Any future determination to declare dividends will be made at the discretion of our board of directors and will depend on our financial condition, results of operations, capital requirements, general business conditions and other factors that our board of directors may deem relevant.

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Market price of common stock

Shares of our Class A common stock commenced trading on the NASDAQ Global Select Market under the symbol “ALTR” on November 1, 2017. Prior to that date, there was no public market for our Class A common stock. The following table summarizes the high and low sale prices of our common stock as reported by the NASDAQ Global Select Market:

On June 6, 2018, the last reported sale price of our Class A common stock on the NASDAQ Global Select Market was $36.29 per share. As of March 31, 2018, there were approximately 465 registered stockholders of record of our Class A common stock and 5 registered stockholders of record of our Class B common stock. This figure does not reflect the beneficial ownership or shares held in nominee name.

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Capitalization

The following table shows our cash and cash equivalents and our capitalization as of March 31, 2018 on:

You should read this table together with the sections of this prospectus titled “ Summary Historical Consolidated Financial and Other Data,” and “Description of Capital Stock” and the section titled “Management’s Discussion and Analysis of Financial Condition and Results of Operations” and our unaudited consolidated financial statements and related notes in our Quarterly Report on 10-Q for the quarter ended March 31, 2018, which is incorporated by reference in this prospectus.

The total number of shares of our common stock to be outstanding set forth in the table above is based on 27,357,012 shares of our Class A common stock and 36,507,676 shares of our Class B common stock, outstanding as of March 31, 2018, and excludes:

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Dilution

If you invest in our Class A common stock in this offering, your ownership interest will be diluted immediately to the extent of the difference between the public offering price per share of our Class A common stock and the as adjusted net tangible book value per share of our Class A common stock immediately after this offering.

Net tangible book value per share is determined by dividing our total tangible assets less our total liabilities by the number of shares of our common stock outstanding. Our historical net tangible negative book value as of March 31, 2018, was $(19.1) million, or $(0.30) per share. After giving effect to (i) the sale and issuance of 3,292,580 shares of our Class A common stock offered by us in this offering based on the public offering price of $35.00 per share, after deducting the underwriting discounts and commissions and estimated offering expenses payable by us, and (ii) the issuance of 257,000 shares of our Class A common stock to certain selling stockholders upon the exercise of stock options in order to sell such shares in this offering, including net proceeds of $0.5 million received by us in connection with the exercise of such options, our as adjusted net tangible book value as of March 31, 2018, would have been $90.7 million, or $1.35 per share. This represents an immediate increase in net tangible book value of $1.65 per share to our existing stockholders and an immediate dilution in net tangible book value of $33.65 per share to investors purchasing shares of our Class A common stock in this offering at the public offering price.

The following table illustrates this dilution on a per share basis to new investors:

The total number of shares of our common stock to be outstanding set forth in the table above is based on 27,357,012 shares of our Class A common stock and 36,507,676 shares of our Class B common stock, outstanding as of March 31, 2018, and excludes:

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Principal and selling stockholders

The following table sets forth certain information with respect to the beneficial ownership of our common stock as of May 15, 2018, which is referred to as the Beneficial Ownership Date, and as adjusted to reflect the sale of our Class A common stock offered by us and the selling stockholders in this offering assuming no and full exercise of the underwriters’ option to purchase additional shares from us, for:

The amounts and percentage of shares of common stock beneficially owned are reported on the basis of regulations of the SEC governing the determination of beneficial ownership of securities. Under the rules of the SEC, a person is deemed to be a “beneficial owner” of a security if that person has or shares “voting power,” which includes the power to vote or to direct the voting of such security, or “investment power,” which includes the power to dispose of or to direct the disposition of such security. In computing the number of shares beneficially owned by a person and the percentage ownership of that person, we have deemed shares of our Class A common stock subject to options that are currently exercisable or exercisable within 60 days of May 15, 2018, and shares of our Class A common stock that are reserved for settlement of RSUs that are vested or will be vested within 60 days of May 15, 2018, to be outstanding and to be beneficially owned by the person holding the option for the purpose of computing the percentage ownership of that person but have not treated them as outstanding for the purpose of computing the percentage ownership of any other person.

The percentages reflect beneficial ownership immediately prior to and immediately after the completion of this offering as determined in accordance with Rule 13d-3 under the Exchange Act and are based on 29,049,457 shares of our Class A common stock and 35,846,152 shares of our Class B common stock outstanding as of the Beneficial Ownership Date. We have based percentage ownership of our common stock after this offering assuming no exercise of the underwriters’ option to purchase additional shares of Class A common stock from us on 34,443,005 shares of our Class A common stock outstanding as of the Beneficial Ownership Date and on 34,170,732 shares of our Class B common stock outstanding as of the Beneficial Ownership Date, reflecting the sale of 5,600,000 shares of our Class A common stock by us and the selling stockholders in this offering (including the shares to be issued upon the exercise of options to purchase shares of our Class A common stock and sold in this offering).

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Except as noted below, the address for all beneficial owners in the table below is c/o Altair Engineering Inc. at 1820 E Big Beaver Road, Troy MI 48083.

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Certain relationships and related party transactions

In addition to the executive officer and director compensation arrangements discussed in the sections titled “Executive compensation” and “Director Compensation” in our definitive proxy statement on Schedule 14A, which is incorporated by reference in this prospectus, the following is a description of each transaction since January 1, 2015 and each currently proposed transaction in which:

Redemptions of our old Class B and old Class A common stock

On November 10, 2012, we redeemed 525,600 shares of our common stock held by George J. Christ, one of the holders of more than 5% of our outstanding capital stock, for an aggregate purchase price of $1,971,000, payable in 36 monthly installments with an interest rate of 1% per year, commencing on December 10, 2012 and ending on November 10, 2015.

On December 10, 2012, we redeemed 525,600 shares of our common stock held by Mark Kistner, Trustee of the Mark Kistner Revocable Living Trust dated January 22, 1998, as amended, one of the holders of more than 5% of our outstanding capital stock, for an aggregate purchase price of $1,971,000, payable in 36 monthly installments with an interest rate of 1% per year, commencing on January 10, 2013 and ending on December 10, 2015.

On May 1, 2015, we redeemed 200,000 shares of our common stock held by Upali Fonseka, who prior to our initial public offering held more than 5% of our outstanding capital stock, for an aggregate purchase price of $711,000 payable in 12 equal quarterly installments with an interest rate of 1% per year, commencing on August 1, 2015 and ending on May 1, 2018.

On January 1, 2016, we redeemed 175,200 shares of our common stock held by the Mark Kistner Trust, one of the holders of more than 5% of our outstanding capital stock, for an aggregate purchase price of $671,892, which was paid in 12 equal monthly installments commencing on January 10, 2016.

On January 1, 2016, we redeemed 175,200 shares of our common stock held by George J. Christ and Deborah M. Christ, Trustees of The Christ Revocable Trust dated May 8, 2015, one of the holders of more than 5% of our outstanding capital stock, for an aggregate purchase price of $671,892, which was paid in 12 equal monthly installments commencing on January 10, 2016.

On June 6, 2016, we redeemed 137,268 shares of our common stock held by Dennis Zuccaro, Trustee of the Dennis & Kathleen Zuccaro Trust dated January 19, 1987, which prior to our initial public offering held more than 5% of our outstanding capital stock, for an aggregate purchase price of $499,998, which was paid in two installments of $250,000 and $249,998, on August, 1, 2016 and December 1, 2016, respectively.

On December 1, 2016, we redeemed 113,388 shares of our common stock held by George J. Christ and Deborah M. Christ, Trustees of The Christ Revocable Trust dated May 8, 2015, one of the holders of more than 5% of our outstanding capital stock, for an aggregate purchase price of $540,010, which was paid in nine equal monthly installments commencing on January 10, 2017.

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On December 1, 2016, we redeemed 59,108 shares of our common stock held by James E. Brancheau & Paula M. Brancheau JTWROS for an aggregate purchase price of $267,316, which was payable in 11 installments and accrued interest at a rate of 1% per month. We paid the first installment of $22,464 on January 10, 2017 and paid the remainder of the purchase price, plus interest, in ten equal monthly installments beginning on February 10, 2017.

Altair Bellingham, LLC lease and acquisition

On December 28, 2000, we signed a real estate lease with Altair Bellingham LLC, or Bellingham, a Michigan limited liability company, of which the James R. Scapa Declaration of Trust dated March 5, 1987, as amended, or the Scapa Trust, owned 39.56% of the outstanding membership interest, Mark E. Kistner owned 24.18% of the outstanding membership interest and George J. Christ owned 36.26% of the outstanding membership interest, to lease the building that we use as our world headquarters in Troy, Michigan. James R. Scapa, the chairman of our board of directors and our chief executive officer, is the trustee of the Scapa Trust. Mr. Kistner and Mr. Christ are each holders of more than 5% of our outstanding capital stock. Pursuant to the terms of the lease, we paid Bellingham rent of $577,501 and $416,225 in each of 2014 and 2015, respectively. On October 1, 2015, we acquired 100% of the outstanding equity of Bellingham from the Scapa Trust, Mr. Kistner and Mr. Christ pursuant to a membership interest purchase agreement, or the Bellingham Purchase Agreement. On January 31, 2016, pursuant to the Bellingham Purchase Agreement, we paid the Scapa Trust, Mr. Kistner and Mr. Christ $286,609, $175,182 and $262,701, respectively, as consideration for the purchase of their membership interest in Bellingham.

2013 credit agreement

In 2013, we entered into a credit agreement with JP Morgan Chase Bank, N.A. pursuant to which Bellingham, was an unconditional guarantor until the agreement was amended in 2016.

Investments in Groupknit, Inc.

On October 22, 2015, we acquired 5,000 shares of the common capital stock of Groupknit, Inc., a Michigan corporation, for an aggregate cash purchase price of $80,000. As part of the same investment, our executive officers Mr. Scapa, Mr. Dagg, Mr. Mahalingam, Mr. Chouinard and Mr. Fariello acquired an aggregate of 5,000 shares in Groupknit, Inc. for an aggregate cash purchase price of $80,000. Groupknit, Inc. also issued 15,000 shares each to two individuals in exchange for the contribution by each individual of his rights in the Groupknit Program technology and intellectual property. One such individual is the significant other of Mr. Scapa’s daughter, the chief executive officer of one of our wholly-owned subsidiaries.

On December 27, 2016, we acquired an additional 2,500 shares of the common capital stock of Groupknit, Inc. for an aggregate cash purchase price of $40,000. As part of the same investment, our executive officers, Mr. Scapa, Mr. Dagg, Mr. Mahalingam, Mr. Chouinard and Mr. Fariello, acquired an aggregate of 2,500 shares in Groupknit, Inc. for an aggregate cash purchase price of $40,000.

Indemnification of officers and directors

We have entered into indemnification agreements with each of our current directors and executive officers. These agreements require us to indemnify these individuals to the fullest extent permitted under Delaware law against liabilities that may arise by reason of their service to us, and to advance expenses incurred as a result of any proceeding against them as to which they could be indemnified. We also intend to enter into indemnification agreements with our future directors and executive officers.

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Policies and procedures for related-party transactions

Our audit committee has the primary responsibility for the review, approval and oversight of any “related party transaction,” which is any transaction, arrangement, or relationship (or series of similar transactions, arrangements, or relationships) in which we are, were, or will be a participant and the amount involved exceeds $120,000, and in which the related person has, had, or will have a direct or indirect material interest. Under our related party transaction policy, our management will be required to submit any related person transaction not previously approved or ratified by our audit committee to our audit committee. In approving or rejecting the proposed transactions, our audit committee will take into account all of the relevant facts and circumstances available.

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Description of capital stock

General

The following is a summary of the rights of our Class A and Class B common stock and preferred stock and certain provisions of our certificate of incorporation and bylaws. This summary does not purport to be complete and is qualified in its entirety by the provisions of our certificate of incorporation and bylaws, copies of which are filed as exhibits to the registration statement of which this prospectus is a part.

Our authorized capital stock consists of 600,000,000 shares, with a par value of $0.0001 per share, of which:

Our board of directors is authorized, without stockholder approval, except as required by the listing standards of the Nasdaq Global Select Market, to issue additional shares of our capital stock.

As of March 31, 2018, we had 27,357,012 shares of our Class A common stock and 36,507,676 shares of our Class B common outstanding, held by approximately 470 stockholders of record and no shares of preferred stock outstanding.

Class A and Class B common stock

Voting rights

Holders of our Class A common stock and Class B common stock have identical rights, provided however that, except as otherwise expressly provided in our certificate of incorporation or required by applicable law, on any matter that is submitted to a vote of our stockholders, holders of Class A common stock are entitled to one vote per share of Class A common stock and holders of Class B common stock are entitled to 10 votes per share of Class B common stock. Holders of shares of Class A common stock and Class B common stock vote together as a single class on all matters (including the election of directors) submitted to a vote of stockholders, unless otherwise required by Delaware law or our certificate of incorporation. Delaware law could require either holders of our Class A common stock or Class B common stock to vote separately as a single class in the following circumstances:

Under our certificate of incorporation, we may not issue any shares of Class B common stock, other than those shares issuable upon exercise of options, warrants, or similar rights to acquire Class B common stock outstanding immediately prior to the filing of the certificate of incorporation with the Secretary of State of the State of Delaware and in connection with stock dividends, unless that issuance is approved by the affirmative vote of the holders of a majority of the outstanding shares of Class A common stock and Class B common stock, each voting separately as a class.

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We have not provided for cumulative voting for the election of directors in our certificate of incorporation. Our certificate of incorporation and bylaws provide for a classified board of directors consisting of three classes of approximately equal size, each class serving staggered three-year terms. Only one class will be elected at each annual meeting of our stockholders, with the other classes continuing for the remainder of their respective three-year terms.

No preemptive or similar rights

Our classes of common stock are not entitled to preemptive rights and are not subject to conversion, redemption or sinking fund provisions.

Economic rights

Except as otherwise expressly provided in our certificate of incorporation or required by applicable law, shares of Class A common stock and Class B common stock have the same rights and privileges and rank equally, share ratably and are identical in all respects as to all matters, including, without limitation those described below.

Dividends and distributions

Subject to preferences that may apply to any shares of preferred stock outstanding at the time, the holders of Class A common stock and Class B common stock are entitled to share equally, identically and ratably, on a per share basis, with respect to any dividend or distribution of cash, property or shares of our capital stock paid or distributed by us, unless different treatment of the shares of each such class is approved by the affirmative vote of the holders of a majority of the outstanding shares of Class A common stock and Class B common stock, each voting separately as a class. In the event a dividend or distribution is paid in the form of shares of Class A common stock or Class B common stock or rights to acquire shares of such stock, the holders of Class A common stock shall receive Class A common stock, or rights to acquire Class A common stock, as the case may be, and the holders of Class B common stock shall receive Class B common stock, or rights to acquire Class B common stock, as the case may be.

Liquidation rights

Upon our liquidation, dissolution or winding-up, the holders of Class A common stock and Class B common stock are entitled to share equally, identically and ratably in all assets remaining after the payment of any liabilities and the liquidation preferences and any accrued or declared but unpaid dividends, if any, with respect to any outstanding preferred stock, unless different treatment of the shares of each class is approved by the affirmative vote of the holders of a majority of the outstanding shares of Class A common stock and Class B common stock, each voting separately as a class.

Change of control transactions

Upon (A) the closing of the sale, transfer or other disposition of all or substantially all of our assets, (B) the consummation of a merger, consolidation, business combination or share transfer which results in our voting securities outstanding immediately prior to the transaction (or the voting securities issued with respect to our voting securities outstanding immediately prior to the transaction) representing less than a majority of the combined voting power of our voting securities or the voting securities of the surviving or acquiring entity, (C) the closing of the transfer (whether by merger, consolidation or otherwise), in one transaction or a series of related transactions, to a person or group of affiliated persons of securities of the Company if, after closing, the transferee person or group would hold 50% or more of our outstanding voting stock (or the outstanding voting stock of the surviving or acquiring entity), (D) any voluntary or involuntary recapitalization, liquidation, dissolution or winding-up, or (E) the issuance by us of voting securities representing more than 2% of our total voting power to a person who held 50% or less of our total voting power immediately prior to such transaction

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and who following such transaction holds more than 50% of our total voting power, the holders of Class A common stock and Class B common stock will be treated equally and identically with respect to shares of Class A common stock or Class B common stock owned by them, unless different treatment of the shares of each class is approved by the affirmative vote of the holders of a majority of the outstanding shares of Class A common stock and Class B common stock, each voting separately as a class.

Subdivisions and combinations

If we subdivide or combine in any manner outstanding shares of Class A common stock or Class B common stock, the outstanding shares of the other class will be subdivided or combined in the same manner, unless different treatment of the shares of each class is approved by the affirmative vote of the holders of a majority of the outstanding shares of Class A common stock and Class B common stock, each voting as a separate class.

Conversion

Each share of our Class B common stock is automatically convertible into one share of our Class A common stock pursuant to the terms of our certificate of incorporation upon the occurrence of certain events. With respect to all beneficial owners, as defined in our certificate of incorporation, of Class B common stock, each share of Class B common stock will convert automatically into one share of Class A common stock upon (i) the date specified by affirmative vote of the holders of at least 66-2/3% of the outstanding shares of Class B common stock, (ii) the executive holder, as defined in our certificate of incorporation, is neither (x) an executive officer of the company nor (y) a director of the company, (iii) the date on which the executive has beneficial ownership of less than 10% of the capital stock of the company, or (iv) the 12 year anniversary of the date of filing of our certificate of incorporation in connection with our initial public offering.

With respect to each individual beneficial owner of Class B common stock, each share of Class B common stock held by a beneficial owner will convert automatically into one share of Class A common stock (i) at any time at the option of such owner, (ii) upon any transfer, whether or not for value, except for certain transfers described in our certificate of incorporation, including, without limitation, transfers from a founder, as defined in our certificate, to another founder, or certain permitted transferees, or (iii) in the event any beneficial owner owns shares of Class B common stock constituting less than 3% of the outstanding shares of Class B common stock.

Each share of Class B common stock held by all beneficial owners of Class B common stock, except the executive holder, will convert automatically into one share of Class A common stock in the event the key holders, as defined in our certificate of incorporation, beneficially own, in the aggregate, more shares of Class B common stock than the executive holder in which event the only holder of Class B common stock will be the executive holder.

In addition, upon the death or incapacity of a beneficial owner of Class B common stock, other than the executive holder, each share of Class B common stock held by such beneficial owner will convert automatically into one share of Class A common stock, immediately upon such death or incapacity, except, with respect to the key holders, such automatic conversion will occur on the date which is nine (9) months after the date of such death or incapacity. Upon the death or incapacity of the executive holder, each share of Class B common stock held by all beneficial owners of Class B common stock will convert automatically into one share of Class A common stock on the date which is nine (9) months after the date of such death or incapacity.

Preferred stock

Pursuant to our certificate of incorporation, our board of directors will have the authority, without further action by the stockholders, to issue from time to time up to 45,000,000 shares of preferred stock in one or more series. Our board of directors may designate the rights, preferences, privileges and restrictions of the

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preferred stock, including dividend rights, conversion rights, voting rights, redemption rights, liquidation preference, sinking fund terms and the number of shares constituting any series or the designation of any series. The issuance of preferred stock could have the effect of restricting dividends on the Class A and Class B common stock, diluting the voting power of the Class A and Class B common stock, impairing the liquidation rights of the Class A and Class B common stock or delaying, deterring or preventing a change in control. Such issuance could have the effect of decreasing the market price of the Class A and Class B common stock. We currently have no plans to issue any shares of preferred stock.

Option and restricted stock unit awards

Our compensation committee has general administrative authority with respect to our equity compensation plans. Although stock options granted under our 2001 NQSO Plan, our 2001 ISO and NQSO Plan and 2012 Plan remain outstanding, no options or other awards are available for issuance under those plans. Instead, any subsequent grants of RSU’s, stock options or other awards will be made under our 2017 Plan.

As of March 31, 2018, under our 2017 Plan, we had outstanding RSUs covering an aggregate of 58,295 shares of our Class A common stock and 8,104,971 shares of our Class A common stock were available for future grant. The number of shares available for issuance under our 2017 Plan will also include an annual increase on the first day of each fiscal year in an amount not to exceed three percent of the outstanding shares of all classes of common stock as of the last day of our immediately preceding fiscal year. We plan to issue RSU awards in early 2019 to senior executives, sales team members and other designated employees based upon a combination of factors, including individual and corporate performance related to fiscal 2018. The determination of the actual number of the individual and aggregate number of RSUs to be awarded and subject to a four-year vesting period is expected to be finalized in early 2019, and remains subject to compensation committee approval at that time. Based upon current expectations, potential dilution is expected to be below 1% for this future award of RSUs. This program is targeted to specific groups of employees identified above. The anticipated dilution from that program does not include RSU’s and stock options that may be granted to our chief executive officer, to employees of entities we may acquire, as retention grants to existing employees or as grants to new employees.

As of March 31, 2018:

Our board of directors generally has the power to amend or terminate our equity compensation plans, subject to limitations on the extent to which previously granted awards may be impacted. In the event of a change of control, our board of directors has authority, that varies from plan-to-plan, to provide for limited or full vesting. With respect to awards granted under the 2017 Plan, in the event of a change of control, each outstanding award will be treated as the administrator determines, except that if a successor corporation or its parent or subsidiary does not assume or substitute an equivalent award for any outstanding award, then such award will fully vest, all restrictions on such award will lapse, all performance goals or other vesting criteria applicable to such award will be deemed achieved at 100% of target levels, and such award will become fully exercisable, if applicable, for a specified period prior to the transaction. The award will then terminate upon the expiration of the specified period of time.

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Anti-takeover effects of Delaware law and our certificate of incorporation and bylaws

Our certificate of incorporation and bylaws contains provisions that could have the effect of delaying, deferring or discouraging another party from acquiring control of us. These provisions and certain provisions of Delaware law, which are summarized below, could discourage takeovers, coercive or otherwise. These provisions are also designed, in part, to encourage persons seeking to acquire control of us to negotiate first with our board of directors. We believe that the benefits of increased protection of our potential ability to negotiate with an unfriendly or unsolicited acquirer outweigh the disadvantages of discouraging a proposal to acquire us.

Dual class stock.    As described above under the heading “Description of capital stock—Class A and Class B common stock—Voting rights,” our certificate of incorporation provides for a dual class common stock structure, which provides our founders and certain others with significant influence over all matters requiring stockholder approval, including the election of directors and significant corporate transactions, such as a merger or other sale of our company or its assets.

Issuance of undesignated preferred stock.    As discussed above under the heading “Description of capital stock—Preferred stock,” our board of directors has the ability to designate and issue preferred stock with voting or other rights or preferences that could deter hostile takeovers or delay changes in our control or management.

Limits on ability of stockholders to act by written consent or call a special meeting.    Our certificate of incorporation provides that our stockholders may not act by written consent. This limit on the ability of stockholders to act by written consent may lengthen the amount of time required to take stockholder actions. As a result, the holders of a majority of our capital stock would not be able to amend the bylaws or remove directors without holding a meeting of stockholders called in accordance with the bylaws.

In addition, our bylaws provide that special meetings of the stockholders may be called only by the chairman of our board of directors, the chief executive officer, the president (in the absence of a chief executive officer) or a majority of our board of directors. A stockholder may not call a special meeting, which may delay the ability of our stockholders to force consideration of a proposal or for holders controlling a majority of our capital stock to take any action, including the removal of directors.

Requirements for advance notification of stockholder nominations and proposals.    Our bylaws establish advance notice procedures with respect to stockholder proposals and the nomination of candidates for election as directors, other than nominations made by or at the direction of our board of directors or a committee of the board of directors. These advance notice procedures may have the effect of precluding the conduct of certain business at a meeting if the proper procedures are not followed and may also discourage or deter a potential acquirer from conducting a solicitation of proxies to elect its own slate of directors or otherwise attempt to obtain control of our company.

Board classification.    Our certificate of incorporation provides that our board of directors is divided into three classes, one class of which is elected each year by our stockholders. The directors in each class serve for a three-year term. Our classified board of directors may tend to discourage a third party from making a tender offer or otherwise attempting to obtain control of us because it generally makes it more difficult for stockholders to replace a majority of the directors.

Election and removal of directors.    Our certificate of incorporation and bylaws contain provisions that establish specific procedures for appointing and removing members of our board of directors. Under our certificate of incorporation and bylaws, vacancies and newly created directorships on our board of directors may be filled only by a majority of the directors then serving on our board of directors. Under our certificate of incorporation and bylaws, directors may be removed only for cause.

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No cumulative voting.    The Delaware General Corporation Law provides that stockholders are not entitled to the right to cumulative votes in the election of directors unless our certificate of incorporation provides otherwise. Our certificate of incorporation and bylaws do not expressly provide for cumulative voting. Without cumulative voting, a minority stockholder may not be able to gain as many seats on our board of directors as the stockholder would be able to gain if cumulative voting were permitted. The absence of cumulative voting makes it more difficult for a minority stockholder to gain a seat on our board of directors to influence our board of directors’ decision regarding a takeover.

Amendment of charter provision.    Any amendment of the above provisions in our certificate of incorporation would require approval by holders of at least two-thirds of our then outstanding Class A and Class B common stock, voting together as a single class.

Delaware anti-takeover statute.    We will be subject to the provisions of Section 203 of the Delaware General Corporation Law regulating corporate takeovers. In general, Section 203 prohibits a publicly held Delaware corporation from engaging, under certain circumstances, in a business combination with an interested stockholder for a period of three years following the date the person became an interested stockholder unless:

Generally, a business combination includes a merger, asset or stock sale, or other transaction resulting in a financial benefit to the interested stockholder. An interested stockholder is a person who, together with affiliates and associates, owns or, within three years prior to the determination of interested stockholder status, did own 15% or more of a corporation’s outstanding voting stock. We expect the existence of this provision to have an anti-takeover effect with respect to transactions our board of directors does not approve in advance. We also anticipate that Section 203 may discourage attempts that might result in a premium over the market price for the shares of Class A common stock held by stockholders.