A key for use in a cylinder lock with a rotatable key plug having a profiled keyway including a longitudinal profile rib at a side wall of the keyway. The key has an elongated, substantially flat key blade (120; 120′) having a longitudinal profile groove (122; 122′) extending along at least a portion of the length of the key blade. The groove has an undercut portion (129) adjacent to a ridge portion (128, 128′), the outside of which forms part of a side surface (123; 123′) of the key blade and the inside of which comprises a side wall portion (126; 126′) being inclined and facing the bottom wall (124; 124′) of the groove. The undercut portion (129; 129′) of the groove is expanded, at its innermost part adjacent to the inclined side wall portion (126; 126′), into a longitudinally extending pocket (135; 135′).