The profile of a familiar type of key is shown in Fig. la. The shape of the key profile is determined by the depths of successive cuts. Each cut is of wedge shape. The centres of the cuts lie at N regular well-defined positions. In the key shown there are 6 cuts (N = 6). The depths d of these cuts, which distinguish one key from another, must have one of a set D of well-defined values. If we regard a cut of zero depth as defined by d = 1, the key shown may have values d = 1, 2, …, 6, 7 (D = 7).