1. A method of enabling a user to instruct a computer system to perform an operation via: an interface surface including an interface, the interface including visible graphical information and substantially invisible coded data, the coded data comprising a plurality of substantially undifferentiated marks positioned relative to a predetermined pattern; and a sensing device which, when placed by the user into an operative position relative to the interface surface to designate at least some of the graphical information, senses at least some of the coded data and generates indicating data indicative of the operation using the sensed coded data; the method including the steps of, in the computer system: receiving the indicating data from the sensing device; and performing the operation indicated by the indicating data; wherein the graphical information designated by the user is at least partially indicative, to the user, of the operation.