# Matching

See how your eligible order receives priority and becomes one or more fills.

**Matching** decides which eligible buy and sell orders trade, at what price, and for how much. It begins after **[Admission](https://testnet.polyester.com/docs/user-docs/polyengine/admission)** accepts your order and reserves the balance it may use.

Each trading pair has its own order book. Deposits from different supported chains trade through the same unified asset, so the source chain does not create a separate market.

## Price comes first, then time

PolyEngine gives priority in two steps:

1. **Best price first.** Buyers receive the lowest eligible sell price, while sellers receive the highest eligible buy price.
2. **Earlier order first.** When several orders share the same price, the order that arrived earlier receives priority.

Your limit price is the worst price you accept, not necessarily the price you receive. A buy can fill at its limit price or lower, and a sell can fill at its limit price or higher.

## Maker and taker fills

The order already resting on the book is the **maker**. The incoming order that trades against it is the **taker**. The applicable maker fee, maker rebate, or taker fee is calculated for each fill.

Maker or taker status belongs to the fill, not permanently to the order. A limit order can take liquidity immediately and later provide liquidity with its remaining amount. **Post-only** prevents that mixed result by rejecting a limit order that would immediately trade as a taker.

See **[Trading Fees](https://testnet.polyester.com/docs/user-docs/fees-and-vip/trading-fees)** for fee and rebate rates.

## Partial fills and remaining amounts

One order can trade against several orders or prices. Each match creates a separate fill, and Polyester combines those fills to show your total filled amount and average execution price.

What happens to the unfilled amount depends on the instruction:

| Instruction                   | What happens to the unfilled amount                                        |
| ----------------------------- | -------------------------------------------------------------------------- |
| **Good Til Canceled (GTC)**   | It remains on the order book until it fills or you cancel it.              |
| **Immediate Or Cancel (IOC)** | It is canceled after all immediately available matches complete.           |
| **Fill Or Kill (FOK)**        | The entire order fills immediately or no part of it fills.                 |
| **Market**                    | It trades immediately within price protection, then cancels any remainder. |

FOK checks the eligible liquidity across the complete order book, not only the rows currently visible in the Order Book widget.

## Prevent self-trading

Self-trade prevention applies when two orders from the same Polyester Account ID would otherwise match:

- **Expire Maker** cancels the resting order.
- **Expire Taker** cancels the incoming order.
- **Expire Both** cancels both orders.

When you do not choose a mode, Polyester uses **Expire Maker**. Different Subaccounts have different account IDs, so orders from two Subaccounts can match normally even when they belong to the same Main Account.

## Canceling a working order

A working order remains active if you close your browser, sign out, or lose your connection. It continues following its price, priority, and time-in-force instructions until it fills, is canceled, or otherwise ends.

A fill can complete while your cancellation is being processed. Any completed fill remains final, while only the amount still open is canceled.

> **Every fill moves to Settlement**
>
> Matching creates the price, amount, maker and taker roles, and order references for each fill. **[Settlement](https://testnet.polyester.com/docs/user-docs/polyengine/settlement)** then applies the asset, fee, rebate, and balance changes to your account.