The =paracentric system=, sometimes called the _duplex system_, consists in having two separate cylinders to each lock, as illustrated in Fig. 47. One of these cylinders, generally the upper one, is operated by the change key, and the lower one, by the master key. The interior construction of the lock is so devised that each key performs the necessary function of operating the same bolt, so that the individual lock may always be opened by the change key, and all locks of the series by the master key. This latter system of lock construction is more expensive than the one-cylinder type, but it possesses an advantage in that it provides greater security when a limited number of locks are grouped in one series. =41.= Owners or officials of large office buildings and industrial works now avail themselves of the master-key system just explained. They are able to obtain mortise locks, rim locks, and padlocks, all arranged to operate with a master key, in one series. In fine residence work, this system is also adopted. Such a system of locking is easily recognized as convenient, especially where subordinates are held responsible for certain rooms or departments to which they, individually, have access, as all of the rooms or departments may be entered by the manager or superintendent by the aid of the master key. A series of master-key locks may also be “submaster-keyed” by dividing it into subordinate groups. In such a case, each group is operated by a master key of its own, and all the subordinate groups are controlled by a grand master key. For example, a six-story office building could be furnished throughout with locks having non-changeable keys; the doors of each floor could be operated separately by a master key; and the doors of the entire building could be operated by a grand master key. The convenience of such a system is readily apparent in large buildings where each janitor is responsible for a certain floor, and where the head janitor, manager, or owner, has control of all the locks through the grand master key. In some instances, for additional security and for special work, the corrugations in the keyway are changed in shape so that the manufacturers’ regular type of key will not enter the keyway, or plug, thus allowing no chance for the regular type of key to operate the lock. =42. Locks for Residence Use.=—The =front-door lock= is distinctly a lock having two bolts; namely, a _dead bolt_ and a _latch bolt_. The latter is operated by the knob, and is so arranged that, by means of stop-work in the lock front, the outer knob may, at will, be set so as not to operate the latch, the latch bolt being operated only from the outside by a key. [Illustration: FIG. 48] The cylinder type of front-door lock illustrated in Fig. 44 is the best lock for this purpose, as it provides the greatest security. This lock is operated by a convenient key of the Yale type, which throws both latch and dead bolt with one insertion into the cylinder.