In integrated circuits, the stepping level or revision level is a version number that refers to the introduction or revision of one or more photolithographic photomasks within the set of photomasks that is used to pattern an integrated circuit. The term originated from the name of the equipment ("steppers") that exposes the photoresist to light.Integrated circuits have two primary classes of mask sets: firstly, "base" layers that are used to build the structures, such as transistors, that comprise circuit logic and, secondly, "metal" layers that connect the circuit logic.