1. A computer system for trading instruments between traders, the computer system comprising: a plurality of order input devices, at least one of said order input devices for entering into the computer system a joint execution order comprising two or more linked orders; means for matching and executing joint execution orders, the means for matching and executing performing only one of: executing, as separate trades, all of the linked orders of the joint execution order; and rejecting all of the linked orders of the joint execution order.