In office buildings, hotels and lodging houses, it is desirable that the person in charge shall be able to enter any room when not occupied by means of a single key that will operate all of the locks, while the regular keys shall not be interchangeable. Locks made on this principle are termed " master-keyed locks," the master key operating all of the locks in the set, while the "change key" will only operate the lock for which it is made.

All of the better grades of mortise knob tumbler locks can be had master keyed, at a slight advance in price, and most cylinder escutcheon locks can be had master keyed. The method adopted for master keying differs with different makes and grades of locks. In the cheaper grades master keying is accomplished by merely introducing different wards or obstructions either in the keyhole or in the path of the key, and making a skeleton key that will pass these obstructions. Such locks offer little security. Another method has been to have different keyholes for the master key and change key.

The best grades of tumbler locks are master keyed by means of auxiliary tumblers, which, when raised to the proper height by the master key, also set the remainder of the tumblers and enable the bolt to be shot, while the change key does not act on these auxiliary levers and must be bitted for each lock to correspond with the combination of levers, and hence will not interchange. Each lock, therefore, has all the security of an ordinary three-lever lock. Such locks, however, require careful adjustment and excellent workmanship, and are therefore somewhat expensive. Master keyed tumbler locks are made in sets of from 200 to 300, and by using different sets with different master keys, as many as 4,000 changes may be had.

Cylinder escutcheons may also be master keyed in sets and with a grand master key.

The system of master keying offering the greatest security, in the opinion of the author, is the Yale duplex master key system, which consists in every instance of two Yale escutcheons acting upon one and the same bolt. This gives two keyholes to each lock, the lower ones for the master key being set to the same combination throughout the suite, while the upper ones are all different. By this system all of the security of the Yale lock is retained, while a single master key can be made to pass a series of locks of an almost indefinite number of changes.

Hotel Locks. - The especial requirements of hotel locks are that they shall be master-keyed and so arranged that while they may be secured from the inside the occupant cannot possibly be locked in. The best hotel tumbler locks have three bolts, a latch bolt operated both sides by knobs, a key bolt operated from outside only by both the "change" or room key, and the master key and a dead bolt operated from the inside by a thumb piece. The dead bolt cannot possibly be operated from without, and when the door is locked from within a curtain is thrown over the outer keyhole so that the key cannot be inserted.

For the cheaper class of hotels a common three-bolt chamber lock, master keyed, or a common knob lock and latch, master keyed and locked from the inside by the change key only, are commonly used.