Polymarket Order Book, Explained

Polymarket runs a central limit order book (the CLOB). Buyers post bids, sellers post asks, each at a price and a size. The book aggregates those resting orders per token, in price levels, per tick. Trades happen when an incoming order crosses the spread.

The book is off-chain: only executions and settlement are written to Polygon. That matters because it means nobody can reconstruct the book from the chain — the past book only exists if someone recorded it while it was live.

Published measurement of sports ladders on this site counts 35 to 36 price levels per side, which is deep enough to watch a market repricing around a goal rather than a single changing best bid.

• timestamp (RFC3339, UTC) and a 1-based sequence number
• bid price, ask price, and mid
• top-of-book bid depth and ask depth, in contracts
• cumulative traded volume for the contract
• market metadata: slug, asset, market type
• a crossed flag that marks books where the best bid exceeds the best ask, left in the data rather than filtered

A price series tells you the mid moved; it does not tell you whether the move was executable. Measured examples on this site show a mid of 0.4995 resting on an empty book, and a repriced 0.500 over a 99.6-cent-wide chasm — the same mid, opposite tradability.

That is the entire case for an order book archive: it records the rows, the depth, and the crossed states so a backtest walks the actual book instead of pretending every fill happened at the mid.

Live books come free from the official API (GET /book). History comes from a recorded archive: this site serves 250ms L2 rows for crypto from 2023 and sports from 2026, with a free Starter tier and paid windows to 30–120 days.

The gap between live and historical is not a pricing decision; it is an engineering fact — the book off-chain must be captured, and capture is what archives sell.

• Pull one market's books at 250ms for an hour and list the distinct levels that appear; Up/Down ladders hug the round grid (0.50, 0.70, 0.95, 0.99...) while sports run 35–36 connected levels per side.
• Count the one-sided frames in your hour; the measured norm is high-90s percent for the Up/Down families, and your own count is the page's argument in practice.
• Label the top-of-book row from a full-level frame and note the executable floor (bid side) and ceiling (ask side); those two levels are the order book you can actually trade against.
• Store the ladder frames with the sequence field; sequence makes the as-of replay exact and that is the whole point of the archive's design.

An order book is a queue of resting limit orders per price level, and Polymarket's is the miniature of it: tick-constrained levels, laddered prices, and mostly one resting side — the shape that makes "buy the book" a walk up a short staircase.

The measured texture — 98.6% of frames pricing at 0.99/0.995, 90%+ one-sidedness, and a full 99.6-cent ladder pass in one second — is what turns the textbook definition into a venue-specific one, and the archive is the place it becomes visible.

The order book page is the foundation: every deeper concept later in the catalog — spread, depth, mid, price bounds — is a reading of the same ladder rows.

The two moves that make book work honest are labeling the executable side and timestamping the frame; both are stored in the archive by design.

Start with a one-hour pull and count the one-sided frames yourself; the measured high-90s share will reproduce against your own data, which is the fastest way to trust the pipeline.

The order book explainer is the base layer; every other concept page assumes its ladder-and-side language.

Pair it with the depth and order-book-history pages for the levels and the sizes, and with the archive pages for the record.

Its one-hour-pull workout is the natural first exercise for anyone new to the catalog.

The order book is the venue's microscope: levels, sides, sizes, and sequences, all recorded at 250ms, and the base for every other reading on this site.

Start the catalog here: pull one hour, count one-side share, label the executable side, and the rest of the vocabulary follows.

• Order book data
• Price history
• Crypto data
• Provider matrix