The trading market automatically matches investors' open orders.. If you post a buy order that partially fulfills a corresponding sell order, the trading market matches them accordingly. It's important to note that the amount of your order that gets filled—whether entirely, partially, or not at all—depends on factors such as other open orders and quantity of shares being offered.
Example:
- Buy Order 1: buy order for 10 shares, at a limit price of $10 entered at 3:00 PM
- Buy Order 2: buy order for 10 shares, at a limit price of $10 at 3:05 PM
- Sell Order 1: sell order for 20 shares at a limit price of $10 at 3:10PM
Trade #1: Buy Order 1, Sell Order 1 for 10 shares at a limit price of $10
Trade #2: Buy Order 2, Sell Order 1 for 10 shares at a limit price of $10
Both Buy Orders 1 & 2 would execute and match, and Sell Order 1 would close.