# Central limit order book

Also written CLOB, order book.

*https://predictionmarkets.tools/glossary/central-limit-order-book · next to Prediction Market Venues*

**Definition:** A matching engine that holds every resting bid and ask for one outcome in one queue, ranked by price and then by time, and pairs them when they cross. The venue never takes the other side and never quotes, so if nobody is resting an order there is no price at all. It is the shape most of this catalogue is built on, and it is not a claim about depth.

Every venue in this catalogue that calls itself an exchange rather than a game is running this,
and the phrase is doing more work than it looks like it is doing. It is not a feature and not a
quality claim. It is a statement about who is on the other side of your trade, and everything
else on the card follows from it.

## How it works

Two queues per outcome. Bids from people willing to pay, asks from people willing to sell, each
sorted by price and then by the moment it arrived, and the venue's only job is to pair the best
bid with the best ask when the two meet. Polymarket US states the consequence in one sentence of
its own documentation: it operates a central limit order book, your order is matched against
another user's order, and the platform does not take the other side of your trades and does not
set prices.

That last clause is the definition. A venue running this shape holds no inventory, publishes no
quote of its own and has no view. [Kalshi](https://predictionmarkets.tools/tools/kalshi), [Polymarket](https://predictionmarkets.tools/tools/polymarket),
[Polymarket US](https://predictionmarkets.tools/tools/polymarket-us) and [Limitless](https://predictionmarkets.tools/tools/limitless) all run it.
[ForecastEx](https://predictionmarkets.tools/tools/forecastex) runs the nearest variant: bids are ranked by price-time priority,
and when a Yes bid and a No bid sum to $1.00 the exchange pairs them, with each side's contract
struck against the clearing house rather than against the other bidder.

### The number on the screen is usually not a price you can take

Polymarket displays the midpoint of the best bid and the best ask, and falls back to the last
traded price once the spread is wider than ten cents. Best bid 34, best ask 40, screen says 37.
You will pay 40 or receive 34, and 37 is the one price at which nobody is standing.

### In a binary market, one side's bid is the other side's ask

Kalshi's order book endpoint returns bids only, for Yes and for No, because a Yes bid at X is a
No ask at a dollar minus X. Read one side of that response, conclude there are no sellers, and
you have missed every seller in the book.

### A book can be empty, and an empty book has no price

This is the property that separates it from everything else in the `liquidity_model` enum. A
curve always quotes, because a curve is a formula. A parimutuel pool never quotes, because there
is nothing to trade until the pool closes. A book quotes when somebody is standing there and
falls silent when nobody is, and the silence is honest: the venue is not withholding a price, it
is declining to invent one.

## What the catalogue records, and the two fields people confuse

`economics.liquidity_model: clob` is set on 46 of the 60 cards here, `amm` on 6, `none` on 8.
Read it as a fact about the **market behind the card**, not about the card's own software: a
Polymarket client, a cross-venue feed or a dashboard sets `clob` because Polymarket runs a book,
which is what makes the data arrive as two-sided ladders with a spread and resting size.

`capabilities.order_book` is a different claim and it is true on 39 cards. It says this tool
*gives you* the book — the ladder, the levels, the resting size — rather than a single price. The
two fields come apart, and the cases where they do are the interesting ones: [Futuur](https://predictionmarkets.tools/tools/futuur),
[Myriad](https://predictionmarkets.tools/tools/myriad) and [Manifold](https://predictionmarkets.tools/tools/manifold) all carry `order_book: true` alongside
`liquidity_model: amm`, because each runs real order types over a curve that catches whatever the
queue does not. Five of the six AMM cards in the catalogue are in that position.

## Why it matters here

**You will read it as a depth claim and size a position on it.** It is not one. A book with one
resting order at 12 cents is a central limit order book, and so is a book with nothing in it.
[Where the liquidity comes from](https://predictionmarkets.tools/guides/where-liquidity-comes-from) has the four numbers that
actually settle the question, and the measured gap between two working venues on the same day was
about three orders of magnitude.

**You will assume you can get out.** On a curve you can always close, at a price. On a book you
can close only if somebody is bidding, and on a thin market at three in the morning nobody is.
Holding to resolution is the default exit on most of this catalogue, not the fallback.

**You will trade at the midpoint in your head.** Every backtest written against a displayed price
rather than against the far side of the spread is optimistic by half the spread on entry and
again on exit, which on the Futuur market we measured at a 6-cent spread is more than three times
the largest trading fee in the sector.

**You will price the fee and forget the book.** The taker fee is published and the spread is not,
and on a thin market the spread is the larger number.
[What a trade actually costs](https://predictionmarkets.tools/guides/what-a-trade-actually-costs) converts the published figures
into the effective fee at fifty cents that every card carries, and then says plainly that on a
thin market it is not the largest part of what you paid.

## Where you will meet this

- [Adjacent](https://predictionmarkets.tools/tools/adjacent.md)
- [Airavat](https://predictionmarkets.tools/tools/airavat.md)
- [Apify prediction-market scrapers](https://predictionmarkets.tools/tools/apify-prediction-market-scrapers.md)
- [Artemis prediction-market metrics](https://predictionmarkets.tools/tools/artemis-prediction-markets.md)
- [CCXT](https://predictionmarkets.tools/tools/ccxt.md)
- [Convexly](https://predictionmarkets.tools/tools/convexly.md)
- [Crypto.com Prediction](https://predictionmarkets.tools/tools/crypto-com-prediction-markets.md)
- [DepthFeed](https://predictionmarkets.tools/tools/depthfeed.md)
- [Polymarket dashboards on Dune](https://predictionmarkets.tools/tools/dune-polymarket-dashboards.md)
- [FinFeedAPI Prediction Markets API](https://predictionmarkets.tools/tools/finfeedapi.md)
- [ForecastEx](https://predictionmarkets.tools/tools/forecastex.md)
- [Futuur API](https://predictionmarkets.tools/tools/futuur-api.md)

## FAQ

### Does "central limit order book" mean the market is liquid?

No. It says who your counterparty is, not how many of them there are. A book with one resting order is still a central limit order book, and an empty one is still a central limit order book with nothing in it. Depth is four separate numbers with dates on them — volume over a stated window, open interest, the spread in cents, and the money resting within two cents of the midpoint.

### Why does a data API card carry liquidity_model, when the tool does not match anything?

Because the field describes the market the tool reads, not the tool. A card for a Polymarket client sets clob because Polymarket runs a book; it is a fact about what the data looks like when it arrives — two-sided ladders, resting orders, a spread — rather than a claim that the library operates an exchange.

### Is ForecastEx a central limit order book?

Its card records clob, and the mechanics are the nearest fit, since bids are ranked by price-time priority, and when a Yes bid and a No bid sum to $1.00 the exchange pairs them. The difference worth knowing is who your contract is with. On Kalshi or Polymarket it is the other user; on ForecastEx each side contracts with the clearing house separately.

### The screen says 37 cents. Why did I pay 40?

Because Polymarket displays the midpoint of the best bid and the best ask, and falls back to the last traded price when the spread is wider than ten cents. If the best bid is 34 and the best ask 40, the displayed 37 is a number nobody is standing at. On any order-book venue, the price you can act on is the far side of the spread, not the middle of it.

## Sources

1. [What is Polymarket US?](https://docs.polymarket.us/getting-started/what-is-polymarket-us) — Polymarket US, read 2026-09-19
2. [Prices & Orderbook](https://docs.polymarket.com/concepts/prices-orderbook) — Polymarket, read 2026-09-19
3. [Orderbook Responses](https://docs.kalshi.com/getting_started/orderbook_responses) — Kalshi, read 2026-09-19
4. [CLOB Overview](https://docs.limitless.exchange/user-guide/clob-overview) — Limitless Exchange, read 2026-09-19

*Last updated 2026-09-21. A reference page, corrected in place — not a dated post.*