In the lever-tumbler type, illustrated in Fig. 39, the tumblers, or wards, are so arranged that each lock can be operated only by its particular key, the keys for all rooms being different and non-changeable; all of the locks, however, can be operated by a key made for the purpose, termed a =master key=. Each lock of this type has two sets of tumblers; one set is operated by its individual key, and the other, being uniform in all locks of the series, is acted on by the master key. Such locks may be obtained either in the cheaper kind, with one tumbler and twelve changes, or in the most intricate styles of hotel locks, with five tumblers and 48,000 changes in one set, and all operated by one _master key_. The cylinder lock of this type is illustrated in Fig. 44, which shows a Yale & Towne, mortise, front-door lock. [Illustration: FIG. 44] =40. Details of the Yale Type of Master-Key Locks.= The =Yale= type of cylinder lock, which is illustrated in Fig. 45 (_a_) and (_b_), is much preferred, on account of the great security it gives and the small key required by it. This lock is made in three systems; namely, the _regular_, the _concentric_, and the _paracentric_, or _duplex_. In the =regular system=, one regular cylinder is controlled by the change and master keys, the pin tumblers being cut in two places, so that the change key brings one set of the abutting planes of the tumbler in alinement with the surface of the cylindrical plug. This plug is arranged so that a separate key is required to operate each lock, the other line of cleavage through the blocks being the same with all locks throughout the series, so that they may be opened with the same key. [Illustration: FIG. 45] [Illustration: FIG. 46] The =concentric cylindrical arrangement= of this type of lock is shown in Fig. 46 (_a_) and (_b_). Here, there is a larger cylinder encircling the key plug. This is known as the master-ring, or larger, plug, and is indicated at _a_. When the change key is inserted in the key plug _b_, the lower series of breaks in the pins comes into alinement with the outer edge of the plug, as shown at (_a_), and allows it to revolve in the master ring, the cam on the inner side at _c_ actuating the lock. When the master key is inserted, the upper series of breaks comes into alinement with the outer edge of the master ring, as shown at (_b_), allowing the plug and the ring to rotate together with the turning of the key, and thus to produce the same result as that caused by the operation of the change key. [Illustration: FIG. 47]