A form of =safe-deposit lock= is illustrated in Fig. 56. This lock is of a new type, and is intended expressly for individual safes, or boxes, rented by the safe-deposit companies. Such locks are nearly always provided with a guard key, which is in charge of a custodian and common to all the locks in the series. Each lock has its own individual, or change, key, however, which differs from every other key in the series. Before a change key can be inserted into its lock, the guard mechanism must be unlocked by the guard key in charge of the custodian. This makes the presence of the latter a necessary preliminary to the unlocking of the box by the renter or his authorized agent. Many styles and sizes of safe-deposit locks are made to meet varying conditions, the lock probably most used being the Yale, or pin-tumbler, variety, in which additional security is obtained by using keys of special proprietary forms. For safe-deposit boxes of large size, the dial lock is often used. =Subtreasury locks= are intended for use on the small safety chests, or “subtreasuries,” often placed within a fireproof safe. These locks, which are illustrated in Fig. 57, are of various types and sizes, and are suitable for use on metal doors of all kinds. [Illustration: FIG. 57] =50. Care and Maintenance of Locks.=—Locks, like other pieces of mechanism, need reasonable care and attention to keep them in the best condition. As they contain moving parts, they are subjected to frictional wear and need occasional lubrication. Most of the friction, and consequently the wear, occurs in the beveled latch bolt, which may be readily lubricated. If the latch bolt is troublesome, its face and back should be cleaned with a cloth moistened with naphtha or kerosene, to remove any dirt. These surfaces should then be wiped with another cloth saturated with machine oil or, better, with vaseline. Any person can perform this simple work, and if it is done once or twice a year, it will keep the bolts in such condition that each door will close easily and quietly at all times. Another cause of trouble is the tendency of the old-fashioned knob screw to become loose, thus allowing the knob to pull from the spindle. Where such spindles are used, it is a good plan to inspect them occasionally and to tighten any loose screws. The best results from lock hardware, however, can be obtained by having all locks and hardware inspected by a lock expert about once a year.