How it works
Submitting an order does not move money on its own. The order sits in the venue’s matching engine until it can be paired with an opposite order. The moment that match happens is the fill. The fill price, quantity, timestamp, and counterparty venue are recorded as the trade. Until the fill, the order is just a request.
Types of fill
- Complete fill: the full quantity executes at one or more prices. Most common for liquid majors.
- Partial fill: only part of the order quantity finds a counterparty at acceptable prices. The rest stays open or is cancelled depending on order type.
- No fill: the order finds no counterparty within its price limits and is rejected or remains pending.
- Fill or kill (FOK): the entire order must fill immediately at the limit price, or the whole thing cancels.
- Immediate or cancel (IOC): fill whatever can fill immediately, cancel the rest.
Example
You submit a limit order to buy 10 standard lots of EUR/USD at 1.0850. The book shows 4 lots available at 1.0850 and 6 lots at 1.0851. A market order would take all 10 (complete fill at average 1.08506). A limit order at 1.0850 only takes the 4 lots at that price; the remaining 6 lots stay open until either the price drops or you cancel.
Why it matters
Fill quality is invisible until you measure it. Two brokers can show the same spread and the same speed in a normal market but fill very differently when liquidity is thin. Track the gap between displayed price and actual fill price across hundreds of trades. That gap is your real cost of trading, not the advertised spread.
