The agreement among one or more managing underwriters and a larger group of underwriters known as the underwriting syndicate to distribute the securities in an offering of securities registered under the Securities Act of 1933, as amended. Also known as the AAU, it governs all the relationships among the underwriters and authorizes the managing underwriters to act on behalf of all the underwriters, including executing the underwriting agreement on their behalf - and to manage the offering.

Most, if not all, investment banks that regularly serve as lead managing underwriters in offerings have adopted master AAUs (MAAUs) that do not have to be separately completed or executed for a particular offering. Each of those firms generally circulates its form MAAU among potential syndicate members, to be signed by them in advance of any specific offering. In the context of a specific offering, the lead manager usually invites the syndicate members to participate in the offering by an invitation notice or "wire." By accepting the invitation, each syndicate member agrees to comply with the lead manager's executed MAAU as it applies to the offering and with the terms of the related underwriting agreement.

A firm's MAAU often follows closely the model MAAU developed by the Securities Industry and Financial Markets Association (SIFMA). Often investment banks just use the model form of MAAU published by SIFMA and available on SIFMA's website.

For more information on the underwriting process, see Practice Note, Underwriting Registered Offering: Overview.