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First-price auction
In a first-price auction the winning bidder pays exactly what it bid, rather than a price set by the runner-up.
Display advertising largely moved to first-price auctions in 2019, when Google Ad Manager switched over. Bidders responded with bid shading — bidding below their true value to avoid overpaying.
A fixed bid ceiling changes the game: when no one can bid above a known maximum, the auction cannot run away from a small buyer.