While the advertising industry mergers that fuel TV show plotlines are hatched over cocktails with Don Draper with nary a deal lawyer in sight, putting together the merger of Omnicom Group and Publicis Groupe—the largest tie-up in ad industry history—required six law firms from around the world.

On Sunday, Paris-based Publicis and New York's Omnicom announced an all-stock combination that will create the world's largest advertising company, with $35.1 billion in market value, $23 billion in combined revenue, and a client roster that includes Bank of America, BMW and Johnson & Johnson. Omnicom and Publicis are currently the second- and third-largest ad firms in the world, behind London-based WPP.