CLOB (Central Limit Order Book)
The CLOB is the matching engine behind both Polymarket and Kalshi. It's what you see when you look at a market: bids on one side, asks on the other, matched when prices cross.
Definition
A central limit order book is an order-matching system that maintains live buy and sell orders for each market, matched by price-time priority.
Plain-English explanation
Every order you place — market or limit — enters the CLOB. If your price crosses an existing counter-order, the trade fills immediately. Otherwise your order sits on the book until someone takes it, or you cancel. Polymarket runs a centralized CLOB off-chain for performance, with trades settled on Polygon. Kalshi runs a fully centralized CLOB on its own infrastructure under CFTC supervision.
Example
You post a limit order to buy YES at $0.45. The best existing sell is at $0.48. Your order joins the book at $0.45. If another trader later places a market sell, they may take your order at $0.45. Until then, you wait.