// GLOSSARY

Post-only order

A limit order that must rest — if it would execute immediately (cross the spread), the venue rejects or re-prices it instead of filling. Guarantees maker treatment: you'll never accidentally pay a taker fee because the book moved as you sent.

A post-only flag turns "I'd like to rest at 43¢" into a guarantee. If, by the time your order arrives, the ask has dropped to 43¢, a normal limit buy would execute instantly — making you a taker with taker fees. Post-only refuses that fill: the order is rejected (or price-adjusted, per venue rules) rather than crossed.

Worked example

You quote both sides of a Kalshi market as a market maker, targeting zero-fee maker economics. News hits, the ask collapses, and your refreshed bid would now cross. Without post-only you take liquidity at the worst possible moment and pay the taker fee; with it, the order bounces and your quoting engine re-prices.

Where it's supported

Post-only (sometimes called "maker-only") is a standard flag on maker-centric venues; availability and exact reject-versus-reprice behavior differ between Kalshi and Polymarket and can change — confirm in each venue's order-type docs. It's essential kit for market making and covered alongside other flags in rate limits and order types.