When you place a limit order close to the market, price can move before it arrives, so it crosses the spread and fills as a taker. Post-only prevents this: if the order would match right away, the exchange rejects or cancels it, and some venues instead move it one tick back. An order that rests on the book fills as a maker when someone trades against it.
Example
BTC's best bid is $60,000 and its best ask $60,001. You place a post-only buy at $60,001; it would match the ask instantly, so it is cancelled. You place it at $60,000 instead; it joins the bid and fills when a seller hits it, paying a 0.02% maker fee instead of 0.05%. On a $50,000 position, that saves $15 on the entry.
Why it matters
For active traders, the gap between maker and taker fees is a large share of total costs. Post-only makes sure the limit orders meant to save fees actually do, and some exchanges pay rebates on maker volume.
Common mistake
Using post-only on orders you truly need filled, such as exits in a fast market. If the order keeps being cancelled because price moves through it, you can miss the exit entirely.
Put it into practice
FAQ
- Why was my post-only order cancelled?
- Because it would have matched an existing order immediately, which would have made you a taker. Place it further from the market so it rests on the book.
- Does post-only guarantee a fill?
- No. It guarantees the fee type, not the fill. Price may never come back to your order.
- Is post-only the same as a limit order?
- It is a limit order with one extra rule: it must rest on the book first. A plain limit can fill as a taker if it crosses the spread.