Abstract: A device for decoding a cylinder lock of the wafer type includes a main portion having a shank similar to an uncut blank for a key for a particular lock and a thin feeler which slides along one side of the portion resembling a key blank. A notch is defined in the portion resembling a key blank, to receive a tumbler of the lock in a normal locking position for that tumbler while the feeler is placed into contact with the tumbler. Indicia are provided to indicate the position of the feeler with respect to the notch, in terms of nominal cut depths to which corresponding portions of a key must be cut to produce a key which will operate the lock. In one embodiment the feeler includes a sloping forward face which contacts a tumbler. In another embodiment of the invention the feeler includes a series of steps, each step corresponding to a particular nominal depth of cut for a position of a key to fit that tumbler of the lock into which the feeler is able to fit.