Abstract: A lock has an outer casing and a lock cylinder rotatably mounted therein and having a coaxial keyway of round cross section plus several rows of radial bores to accommodate the tumblers and their springs, and also has a key which has notches on its outside profile for the engagement with the tumbler. The key has an equilateral polygonal bit cross section which matches the cross section of the keyhole in the front wall of the lock casing as well as a coaxial opening in the lock cylinder bottom, these cross sections of the keyhole and bottom opening being angularly offset from one another. The tip of the key bit has ramp surfaces corresponding to the rows of the tumblers, ascending toward the handle of the key and terminating each at one polygon face for the purpose of raising the tumblers, and the rows of notches are disposed between the polygon faces.