For years, ad auctions had a comforting rule: you pay what the runner-up bid, not your own. Then header bidding broke the math, and the whole industry flipped to first-price, where you pay exactly what you bid. Knowing why is knowing how modern CPMs are set.
| Auction | The winner pays | How you bid |
|---|---|---|
| Second-price | The second-highest bid plus a cent | Bid your true value; overpaying is impossible. |
| First-price | Their own winning bid | You must bid below your true value or you overpay. |
Why the industry flipped to first-price
- Header bidding broke the symmetry. Several separate auctions now fed one ad server, so the "second price" inside any one of them was not the true market second price. The guarantee stopped meaning anything.
- Soft floors and last-look made it untrustworthy. An exchange that could see other bids and quietly adjust (last-look), or apply a hidden soft floor, could charge close to the first price while calling it second-price. Buyers stopped trusting it.
Key idea
In first-price, the clearing price is the winning bid itself. In second-price, it is the second-highest bid (plus a small increment). The switch made pricing transparent, and made buyers responsible for not overpaying.
Quick check
Three bids arrive: $4.00, $3.20, $2.10. What does the winner pay under first-price, and under second-price?
Note
First-price is the reason bid shading exists. That is the next lesson.