A key (7) serves for actuating a lock and has a grip, a shank (10) and a key bit (12), on which is arranged the coding of the key (7) in the form of key steps (13, 14, 18). The key bit (12) has key steps (13, 14, 18) on either side, wherein the diameter of their key bit (12), in relation to the key steps (13, 14), which project to the maximum extent in each case, is between 9,5 mm and 11 mm. A lock (1) for such keys (7), having a plurality of tumblers (24) arranged in a lock housing (2), has a rotatably mounted follower (3) which can be actuated via the key (7). This follower (3) is designed in the form of a multifunctional component, as a structural unit for repositioning the tumblers (24) and / or for controlling the locking function.