organ stops can be combined in countless permutations, resulting in a great variety of sounds. a combination action can be used to switch instantly from one combination of stops (called a "registration") to another. combination actions feature small buttons called "pistons" that can be pressed by the organist, generally located beneath the keys of each manual (thumb pistons) or above the pedalboard (toe pistons). the pistons may be "divisional" (affecting only a single division) or "general" (affecting all the divisions), and are either preset by the organ builder or can be altered by the organist. modern combination actions operate via computer memory, and can store several channels of registrations.