How the Polymarket CLOB Order Book Works (October 2026)
Polymarket’s CLOB (Central Limit Order Book) is a hybrid: buy and sell orders are matched off-chain by an operator for speed, then settled on Polygon for finality. Every order is an EIP-712-signed structured message, and a “market order” is simply a limit order priced to execute immediately against resting liquidity.
What is the order book?
The book is the public ledger of every pending order for a market, organized by price level:
- Bids — buy orders, stacked from the highest price down
- Asks — sell orders, stacked from the lowest price up
- Spread — the gap between the best bid and best ask
- Depth — total size resting at each level
A market quoted 64¢/66¢ has a 2¢ spread: you can sell into 64¢ or buy at 66¢ instantly, or rest an order inside the spread and wait to become the new midpoint.
How does matching work?
| Step | Where | What happens |
|---|---|---|
| 1. Sign | Your wallet | Order becomes an EIP-712 message |
| 2. Match | Off-chain operator | Compatible orders paired by price-time priority |
| 3. Execute | Off-chain | Trades execute against the matched book |
| 4. Settle | Polygon | Finality on-chain; balances update |
Because matching is off-chain, fills are fast and free of gas; because settlement is on-chain, the end state is verifiable and non-custodial.
What order types exist?
| Type | Behavior |
|---|---|
| Limit (GTC) | Rests until filled or cancelled |
| Market | Limit priced for immediate execution |
| FOK | Fill everything now or cancel entirely |
| FAK | Fill what’s available now, cancel the rest |
| GTD | Good until a specified date |
| Post-only | Rejected if it would take liquidity |
| Reduce-only | Cannot increase exposure (perps) |
What are complementary books?
In a binary market, the YES book and the NO book are mirror images. Buying YES at 65¢ is economically identical to selling NO at 35¢ — the engine matches across both books, which is why liquidity often appears deeper than either book shows alone. Multi-outcome (neg risk) events extend this: a NO share in one outcome can convert into YES shares across all others, further linking the books.
How do I read depth before trading?
Three numbers to check before sizing up:
- Top-of-book size — how much is available at the best price
- Cumulative depth to your target price — what your average fill will be
- Spread in cents — your instant round-trip cost before any edge
A 1¢ spread with $50 of top-of-book is worse for a $10,000 order than a 3¢ spread with $30,000 of depth: the first forces you to walk the book, the second absorbs you at one price.
FAQ
Q: Is Polymarket an AMM or an order book? A: An order book (CLOB). The matching is off-chain by an operator with on-chain settlement on Polygon — a hybrid-decentralized design, not an AMM pool.
Q: What is a market order on Polymarket? A: A limit order priced to execute immediately against the best resting orders. If depth runs out before your size fills, the remainder stays unfilled or continues at worse prices depending on the order flags.
Q: Do I pay gas for trades? A: No gas per trade — matching is off-chain. Gas applies to on-chain settlements like deposits, withdrawals, and redemptions.
Q: What does post-only do? A: On perps, it rejects your order if it would take liquidity, guaranteeing you remain a maker (and thus eligible for maker rebates).
Q: Why is liquidity deeper than one side of the book suggests? A: Complementary-book matching — YES and NO books are mirrors, and the engine matches across both, effectively pooling liquidity from each side.
Sources
Disclaimer
Not financial advice. Educational purposes only. Updated October 2026.