Method and apparatus for making a plurality of different keys, each constructed to open each rotary cylinder lock in the same series, or sub-series, called "masterkeying". The disclosed masterkeying tool comprises a calibrated plate, having one or more pin grooves, and a key-supporting platform which moves transversely to permit a selected bitting of a test key to register with a selected pin groove on the calibrated plate. Tumbler pins interposed in the pin groove are seated in a selected bitting of the test key and are aligned in the groove to reach the indicated shear line. The process is repeated for corresponding bittings in each of a series of keys, beginning with the key having the highest bitting and ending with the key having the lowest bitting. The aligned pins for each bitting are removed in order and interposed in the corresponding holes in the lock-cylinder plug, so that the corresponding bittings of any one of the test keys interposed through the keyway, will lift up the spring-biased pins above the shear line, releasing the plug to rotate in the cylinder and open the lock. A three piece and a two piece tool embodiment are disclosed.