Second price is a pricing rule for auctions in which the highest bidder wins but pays an amount set by the second-highest bid, usually with a small increment added on top. One number decides who wins. A different number, submitted by somebody else, decides what the winner pays. For roughly fifteen years this was the default way search engines and advertising exchanges charged for a click or an impression, and it exists for a specific reason: if a buyer can never be charged their own bid, there is no obvious incentive to shade that bid downward.

The rule survives in search and retail media. In programmatic display and video it has largely been replaced by first price, where the winner pays exactly what was bid. That migration, concentrated between 2018 and 2019, is the largest change to advertising pricing mechanics in a decade.

How the clearing price is calculated

Take one display impression offered to three demand-side platforms (DSPs), the systems advertisers buy through. Bids come back at $4.20, $2.80 and $1.50, quoted as cost per thousand impressions (CPM). The publisher has set a floor of $1.00. Under a second price rule the $4.20 bidder wins and is charged $2.81, the runner-up bid plus one cent. The $1.39 gap between bid and clearing price is the auction discount, accruing to the buyer rather than the seller.

Floors change the arithmetic. Raise the floor to $3.50 and the winner pays $3.50, because the floor binds above the second bid. Microsoft's documentation for Monetize, the supply-side platform formerly known as Xandr, sets the hierarchy out explicitly: the winner pays the second-highest bid plus one cent, or the deal ask price, or the Estimated Clear Price, a projection drawn from historical bids, whichever is higher. A single-bidder auction has no second bid at all, so the price falls back to whichever reserve applies. That is why publishers running second price auctions had a structural interest in high floors.

Where the rule is declared

In programmatic trading the auction type is a field in the bid request. OpenRTB, the IAB Tech Lab specification governing real-time bidding, carries at on the top-level object, where 1 means first price and 2 means second price plus. The field has read the same way since version 2.3 in November 2014 and is unchanged in 2.6, published April 2022. The Deal object carries its own at override, adding a third value in which the number passed in bidfloor is the agreed deal price rather than a minimum.

The IAB Tech Lab Deals API, released on December 5, 2025, reuses the vocabulary: its Terms object permits dynamic auction, first price, second price plus and fixed price.

Origins

The theory predates the industry by four decades. William Vickrey described a sealed-bid second-price mechanism for a single item in "Counterspeculation, Auctions, and Competitive Sealed Tenders," published in the Journal of Finance in 1961; its appeal was that bidding one's true value is a dominant strategy. GoTo.com, later Overture and then part of Yahoo, introduced automated pay-per-click keyword auctions with pay-your-bid pricing, a format later labelled generalized first price. Sources differ on the date: the presentation accompanying the canonical academic paper places it in 1997, other accounts the 1998 launch. It proved unstable either way, with bidders undercutting each other by a cent in continuous cycles that Benjamin Edelman and Michael Ostrovsky documented in Overture data from June 2002 to June 2003.

Google addressed the instability with AdWords Select in February 2002, building on the observation that a bidder in position i never wants to pay more than one increment above the bid in position i+1. The mechanism became the generalized second price auction, or GSP. Edelman, Ostrovsky and Michael Schwarz formalised it in the American Economic Review in March 2007, and the central finding sat awkwardly against the marketing story built around it: GSP resembles the Vickrey-Clarke-Groves mechanism but has no equilibrium in dominant strategies, and truth-telling is not an equilibrium. Display exchanges adopted the rule as real-time bidding scaled from 2009, and by the mid-2010s it was near universal in open-market display.

Why display abandoned it

Header bidding broke the arrangement. Once publishers ran a parallel auction in the browser and passed a single price into the ad server, an exchange clearing at second price entered that comparison understating its own demand: a buyer bidding $4.20 into a second-price exchange contributed $2.81, while a first-price wrapper bid of $3.00 contributed $3.00 and won.

Sellers had also learned to work the rule. Soft floors, dynamically raised floors and per-buyer pricing let a supply-side platform capture part of the discount the mechanism was supposed to hand to the buyer, and buyers began shading bids in a format where shading was theoretically pointless, with measurable consequences. Agency Hearts and Science tested 15 publishers over three weeks in the second quarter of 2018 and found CPMs 59% higher in first-price than in second-price auctions, according to eMarketer. Bid shading narrowed the gap only slightly, to 54%.

Independent exchanges moved first. Improve Digital declared itself ready for first-price auctions in March 2018. Google went last and largest, announcing in March 2019 that Ad Manager inventory would move to a unified first price auctionunder group product manager Sam Cox. The rollout began the week of September 5, 2019, announced by product management director Jason Bigler, who wrote that testing showed a neutral to positive impact on publishers' total revenue, a claim resting on Google's own data. Buyers were promised the minimum bid price to win once each auction closed, importing second-price information into a first-price world.

According to the complaint Teads filed in the Southern District of New York on August 3, 2026, Project Bernanke deflated the second-highest Google Ads bid and inflated the highest before submitting both into the second-price AdX auction. Bids of $7.06 and $4.71, entered as $18 and $1, clear at $1.00, pay the publisher $0.80 after the take rate and still charge the advertiser $4.71, moving profit from $1.51 to $3.90. Project Bernanke was rewritten as Alchemist in autumn 2019 to survive the move to first price. Google denies wrongdoing and has appealed the liability finding.

Where second price still operates

Search never moved. Google's help documentation describes the pricing rule without ever using the phrase: actual cost-per-click is what is minimally required to clear the Ad Rank thresholds and beat the Ad Rank of the competitor immediately below, and where no competitor sits below, the advertiser pays the reserve price. A Google briefing paper prepared for the United Kingdom's Competition and Markets Authority in 2020, later filed as a Justice Department exhibit, contains a section headed "Randomized General Second-Price Auction," confirming that the deployed mechanism is a randomised variant rather than plain GSP.

Retail media inherited the format. Amazon's sponsored formats are near-universally described in the trade as second price with relevance weighting, though Amazon's own documentation stops short of stating the rule, which makes the widely repeated one-cent increment an inference rather than a disclosure. ChatGPT Ads runs a relevance-weighted second-price mechanism beneath an Ads Manager that opened to all United States businesses on May 5, 2026.

Limitations and disputes

The truthfulness argument does not survive contact with the multi-slot case. Vickrey's result applies to one item; ranked advertising positions are several, and the 2007 formalisation showed GSP is not truthful. Advertisers have always had reason to bid strategically, whatever the incentive-compatibility language in vendor materials suggested.

The information asymmetry cuts both ways. A second price auction hands the auctioneer the winner's full bid while charging less, commercially useful data the buyer never intended to surrender. Where the auctioneer also operates demand, as Google did across AdX and Google Ads, that asymmetry underpinned the case in which a federal court found in April 2025 that the company monopolised the publisher ad server and ad exchange markets. Ari Paparo's account of the period, covered by PPC Land in August 2025, sets First Look and Last Look alongside the pricing rule they exploited.

Adjacent terms

Vickrey auction and VCG. A Vickrey auction is the single-item second-price auction. Vickrey-Clarke-Groves generalises it to multiple items while preserving truthfulness. Neither is what search platforms run.

Generalized second price. The multi-slot variant in which each winner pays the effective bid of the advertiser ranked below, adjusted for quality. Named after the Vickrey mechanism, it behaves differently.

Second price plus. The OpenRTB label, value 2 on the at field. The "plus" denotes the increment above the second bid or floor.

Bid shading. A buy-side technique for lowering a first-price bid toward an estimated clearing price, reconstructing the discount second price supplied automatically.

Recent developments

Auction vocabulary became a standards project. IAB Tech Lab published Programmatic Auction Definitions for public comment on January 29, 2026, a 12-page specification with 15 defined terms, drafted by Jill Wittkopp and Hillary Slattery in response to the Media Rating Council's Digital Advertising Auction Transparency working group. Comments closed on February 27, and the final document arrived on June 26, 2026 as a 12-step workflow paired with those terms. The need for a glossary is instructive: when a DSP and a supply-side platform each say "first price," floors, shading and bid evaluation order can make the two behave differently. Stakes rise as automated buying spreads, since agents inherit whatever rule the seller declares.

Who captures the auction discount was not settled by the format change. Publishers moving from waterfall setups to unified auctions report revenue increases of 15% to 30%, according to DataBeat, a gain attributed to real-time competitive bidding replacing static historical floors rather than to the pricing rule itself.

Timeline

  • 1961: William Vickrey publishes "Counterspeculation, Auctions, and Competitive Sealed Tenders" in the Journal of Finance, describing the sealed-bid second-price mechanism
  • 1997 to 1998: GoTo.com introduces automated pay-per-click keyword auctions with pay-your-bid pricing, later termed generalized first price; sources differ on whether the mechanism dates to 1997 or to the 1998 launch
  • February 2002: Google launches AdWords Select using the generalized second price auction
  • 2002 to 2003: Yahoo and Overture adopt generalized second price
  • March 2007: Edelman, Ostrovsky and Schwarz publish in the American Economic Review, showing that truth-telling is not an equilibrium of GSP
  • 2013: Google's gTrade team launches Project Bernanke, later described in litigation as manipulating bids submitted into the second-price AdX auction
  • November 2014: OpenRTB 2.3 carries the at field with 1 for first price and 2 for second price plus
  • March 2018: Improve Digital declares support for first-price auctions
  • Second quarter of 2018: Hearts and Science tests 15 publishers and records CPMs 59% higher under first price than second price
  • March 6, 2019: Google announces the transition of Ad Manager to a unified first price auction
  • September 5, 2019: Google begins the full rollout of first price auctions to Ad Manager partners
  • April 2022: OpenRTB 2.6 is published, retaining the at field definitions unchanged
  • December 5, 2025: IAB Tech Lab publishes the Deals API, listing second price plus among four permitted pricing types
  • January 29, 2026: IAB Tech Lab opens Programmatic Auction Definitions for public comment
  • June 26, 2026: The final Programmatic Auction Definitions document is released
  • August 3, 2026: Teads files an antitrust complaint describing bid manipulation inside the second-price AdX auction

Summary

Who. Search engines, advertising exchanges and retail media networks operate second price auctions as auctioneers. Advertisers and their demand-side platforms bid into them, and publishers and supply-side platforms set the floors that frequently determine the clearing price. William Vickrey supplied the theory; Google's AdWords Select supplied the first large-scale advertising implementation.

What. A pricing rule under which the highest bidder wins but is charged the second-highest bid plus a small increment, or the applicable floor, whichever is higher. In OpenRTB it is auction type 2, labelled second price plus.

When. Described in 1961, applied to search advertising from February 2002, dominant in programmatic display through the mid-2010s, and displaced by first price across open-market display and video between 2018 and the end of 2019.

Where. Still live in Google Search, Amazon's sponsored formats, newer surfaces including ChatGPT Ads, and in negotiated programmatic deals on platforms such as Microsoft Monetize. Largely absent from open-market programmatic display.

Why. The rule was adopted to stabilise volatile pay-your-bid auctions and to let buyers submit honest valuations. It was abandoned in display because header bidding made second-price clearing prices uncompetitive against first-price wrapper bids, because sellers manipulated floors to recapture the discount, and because the gap between the winning bid and the amount charged gave intermediaries a margin that neither side of the transaction could easily observe.