The invention relates to a lock comprising a lockable knob, formed by a closure tongue (1) non-rotatably mounted on the shaft of a cylinder (2) which rotates in a casing (3) under the action of a knob (4). The knob (4) is mounted on the cylinder (2) and casing (3) such that it can move linearly between two end positions, namely an operating position and a locked position. The casing (3) and the knob (4) include at least two geometrically arranged linear guides (33, 43) which, during the linear movement of the knob (4), are engaged with, or released from one another. In addition, the cylinder (2) and the knob (4) include radial openings (20, 40) which are positioned coaxially in the locked position and offset axially in the operating position. The cylinder (2) and the casing (3) include geometrically arranged cams (22, 32) which limit the rotational movement of the cylinder (2) in the casing (3) between the tongue (1) -opening and -closing positions.