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Every market on Outcome has an order book that shows you who’s willing to buy and who’s willing to sell at what price. Understanding what you’re looking at is the foundation for placing trades intelligently.

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The two sides

The order book has two sides:
  • Bids are orders from people willing to buy a specific outcome at a given price. The bid stack shows the highest prices buyers are offering, sorted from best (highest) to lowest.
  • Asks are orders from people willing to sell. The ask stack shows the lowest prices sellers will accept, sorted from best (lowest) to highest.
Each row in the book shows the price and the total amount of tokens available at that price.

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Best bid, best ask, and spread

The best bid is the highest price anyone is currently willing to pay for the outcome. The best ask is the lowest price anyone is currently willing to sell at. The difference between them is the spread. A tight spread (e.g., bid at 39 cents, ask at 40 cents) means buyers and sellers are close to agreeing on a price, usually a sign of active liquidity. A wide spread (e.g., bid at 30 cents, ask at 50 cents) means there’s significant uncertainty or limited liquidity, and trades will cost more in slippage.

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How your order interacts with the book

When you submit an order, where it lands depends on the price you set relative to the current book:
  • A buy priced at or above the best ask fills immediately at the best available price. This is a taker order (you take an existing offer).
  • A sell priced at or below the best bid fills immediately. Also a taker order.
  • Otherwise, your order rests on the book at your specified price until someone takes it or you cancel. This is a maker order (you make liquidity available for others).
Whether you’re a maker or taker determines what fees apply. See Fees for details.

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One book, two views

For any outcome, the YES and NO order books are merged at the protocol level to share liquidity. Buying YES at a price of 40 cents is the same trade as selling NO at 60 cents: the two are mirror views of one position, and the protocol treats them as one book. In practice this means your order can fill against either side. If you’re buying YES at 40 cents, you can be matched with someone selling YES at 40 cents or with someone buying NO at 60 cents. Liquidity from both views is available to you, which makes the effective book deeper than either side would be alone. You don’t need to manage any of this. Trade whichever side expresses your view, and the matching engine handles the rest.

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Depth

Beyond the best bid and ask, the rest of the book is depth. Depth tells you how much you can trade before the price moves. A market with 100 USDC at the best ask and 100 USDC at the next-best ask has shallow depth, and a large order will walk through both levels. A market with 10,000 USDC stacked at the best ask has deeper liquidity, and the same order would fill at one price.

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Other prices you’ll see

Beyond the order book itself, market pages typically display two related prices:
  • Mid price is the midpoint between the best bid and the best ask. It’s a convenient single-number reference for where the market is currently pricing the outcome.
  • Last trade price is the price of the most recent fill. It tells you what someone actually paid, which can be different from the current mid if the book has moved since the last trade.
For a quick read of where a market is, the mid price is usually the best single number. For tracking activity, the last trade price tells you what’s been happening.