1. A method comprising the steps of: when an electronic trading system matches a first trade command from a first participant to a second trade command from a second participant to transact a financial instrument traded on the trading system, computing and storing in the memory of the electronic trading system a first transaction commission to charge to the first participant and a second transaction commission to charge to the second participant, the two commissions being different from each other, the commission rate charged for the earlier of the first and second trade commands being lower than the commission rate charged to the later of the first and second trade commands; and in the event that the first and second trade commands specify different quantities of the financial instrument, permitting a joiner participant in the electronic trading system to join the transaction to trade the excess of the larger of the two quantities remaining after satisfying the smaller of the first and second trade command, and recording in the memory of the electronic trading system a charge to the joiner participant a commission based on a rate for joiner parties that is different than the commission rates charged for the first and second trade commands.