Post-Only Order

A post-only order is a limit order carrying an instruction that it must be added to the order book and must never execute immediately against an order already resting there. If it would fill on entry, the venue cancels it, rejects it, or on some platforms shifts its price so that it rests instead — which of those happens is a rule of the venue, not a property of the order type. The purpose is fee-related: where a platform charges different rates for adding liquidity and for removing it, the better rate goes to the side that rests, and a post-only flag guarantees the order cannot accidentally land on the more expensive side. Two limits are worth stating. It is a guarantee about how you enter, not about being filled at all, so a post-only order may simply never trade. And it protects against a fee outcome, not against the market moving.