An interface system for enabling a user to instruct a computer system to perform an operation via an interface surface including an interface, the interface including visibly printed graphical information and substantially invisible coded data printed in infra-red ink, the coded data comprising a plurality of substantially undifferentiated marks positioned relative to a predetermined pattern, wherein each mark has a minimum radius and a maximum radius and wherein the length of a mark's minimum radius is greater than 40% of the length of that mark's maximum radius; 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 computer system being configured to: receive the indicating data from the sensing device; and perform 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, and wherein the coded data indicative of the operation is substantially coincident with graphical information indicative of the operation.