Among the most celebrated combination and time locks of the century are those known as the Yale locks, chiefly the inventions of Louis Yale, Jr., of Philadelphia. The Yale double dial lock is a double combination bank or safe lock having two dials, each operating its own set of tumblers and bolts, so that two persons, each in possession of his own combination, must be present at a certain time in order to unlock it. If this double security is not desired, one person alone may be possessed of both combinations, or the combinations may be set as one. In their time locks a safe can be set so as to not only render it impossible to unlock except at a predetermined time each day, but the arrangement is such that on intervening Sundays the time mechanism will entirely prevent the operation of the lock or the opening of the door on that day. Another feature of the lock is the thin, flat keys with bevel-edged notchings, or with longitudinal sinuous corrugations to fit a narrow slit of a cylinder lock. To make locks for use with the corrugated keys machines of as great ingenuity as the locks were devised. In such a lock the keyhole, which is a little very narrow slit, is formed sinuously to correspond to the sinuosities of the key. No other key will fit it, nor can it be picked by a tool, as the tool must be an exact duplicate of the key in order to enter and move in the keyhole. Of late years numerous locks have been invented for the special uses to which they are to be applied. Thus, one type of lock is that for safety deposit vaults and boxes, in which a primary key in the keeping of a janitor operates alone the tumblers or guard mechanism to set the lock, while the box owner may use a secondary key to completely unlock the box or vault. Master, or secondary key locks, are now in common use in hotels and apartment-houses, by which the key of the door held by a guest will unlock only his door, but the master key held by the manager or janitor will unlock all the doors. This saves the duplication and multiplicity of a vast number of extra keys. The value of a simple, cheap, safe, effective lock in a place where its advantages are appreciated by all classes of people everywhere is illustrated in the application of the modern rotary registering lock to the single article of mail bags. Formerly it was not unusual that losses by theft of mail matter were due in part to the extraction of a portion of the mail matter by unlocking or removing the lock and then restoring it in place.