Flat key profile for a lock-key system, in which a key channel corresponding to the key profile is provided in the lock cylinder, - the key profile of the key shank having an upper profile portion facing the key back and a lower profile portion facing the key face, - notches being provided which project into the lower profile portion starting from the key face in the key shank plane, - a triangular, paracentric profile groove that projects above the centre plane of the key shank being provided in the lower profile portion in one side surface of the key shank, - a triangular profile groove (3; 4) arranged above or below the opposite paracentric profile groove being provided on the opposite side surface of the key shank, the profile base of which profile groove is at a horizontal distance from the profile base of the opposite paracentric profile groove, - the arrangement of the paracentric profile groove in relation to the opposite profile grooves being such that an imaginary line which extends perpendicularly with respect to the key shank plane from the point of intersection of the upper or lower leg of the paracentric profile groove with the key side surface to the opposing profile groove is at a vertical distance from an imaginary line which extends from the point of intersection of the upper or lower leg of the opposite profile groove with the key side surface to the paracentric profile groove, - characterised in that - the paracentric triangular profile groove has at least one shoulder in the upper leg with which a shoulder in the lower leg of the opposite upper triangular profile groove is associated, or at least one shoulder in the lower leg with which a shoulder in the upper leg of the opposite lower triangular profile groove is associated, - the shoulder beginning at an imaginary point of intersection of the triangular profile groove with the key outer surface and having a minimum depth of 1/3 of the triangular profile groove. - the key profile of the key shank having an upper profile portion facing the key back and a lower profile portion facing the key face, - notches being provided which project into the lower profile portion starting from the key face in the key shank plane, - a triangular, paracentric profile groove that projects above the centre plane of the key shank being provided in the lower profile portion in one side surface of the key shank, - a triangular profile groove (3; 4) arranged above or below the opposite paracentric profile groove being provided on the opposite side surface of the key shank, the profile base of which profile groove is at a horizontal distance from the profile base of the opposite paracentric profile groove, - the arrangement of the paracentric profile groove in relation to the opposite profile grooves being such that an imaginary line which extends perpendicularly with respect to the key shank plane from the point of intersection of the upper or lower leg of the paracentric profile groove with the key side surface to the opposing profile groove is at a vertical distance from an imaginary line which extends from the point of intersection of the upper or lower leg of the opposite profile groove with the key side surface to the paracentric profile groove, - characterised in that - the paracentric triangular profile groove has at least one shoulder in the upper leg with which a shoulder in the lower leg of the opposite upper triangular profile groove is associated, or at least one shoulder in the lower leg with which a shoulder in the upper leg of the opposite lower triangular profile groove is associated, - the shoulder beginning at an imaginary point of intersection of the triangular profile groove with the key outer surface and having a minimum depth of 1/3 of the triangular profile groove.