Abstract: The invention relates to a lock-key system comprising a flat key (1), which is insertable into the keyway of a cylinder core (3), wherein grooves (4) and ribs (5) inside the keyway are assigned to corresponding ribs (5a) and grooves (4a) located on both flat sides of the key (1), wherein the key is subdivided from the key spine (6) to the key facing into different profile sections. The middle profile section is assigned to at least one groove (4) on one outer surface of the keyway (2) and to a rib (5) on the opposite outer surface of the keyway. Both the groove (4) as well as the rib (5) comprise a flank (4?, 5?), which extends perpendicular or substantially perpendicular to the center plane of the key or of the keyway, and comprise a flank (4?, 5?), which extends slantingly to this center plane.