Omnicom’s historic acquisition of rival Interpublic Group (IPG) was finalized Wednesday . Together, the groups form the largest advertising holding company on the globe, with combined annual revenues that could surpass $26 billion next year, according to SEC filings.

The deal is likely to reconstitute a profitable—and controversial—part of agency economics: principal-based media buying, a practice that entails buying media in bulk and reselling it to clients at a markup. Omnicom is one of the leaders in principal buying, and its acquisition of IPG is set to vault the new company even further ahead in the practice, analysts and buyers told ADWEEK.

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