This term is now quite generally used to designate those locks in which the bolt or latch, or both, are operated by means of a cylinder escutcheon, which is really separate from the lock proper. The first cylinder lock or escutcheon was invented by Linus Yale about the year i860, and for a number of years the "Yale" lock was the only cylinder lock on the market. The great success of this lock has led to the adoption of somewhat similar escutcheons by other lock manufacturers, so that there are now four or five cylinder locks in common use.

The original Yale lock had a small flat key and a small narrow slit for the keyhole. About the year 1880 a corrugated key and keyhole was introduced which further increased the security of the lock and the possible number of changes. This has in turn been superseded by the Yale Paracentric escutcheon, which represents the highest development in key locks.

The construction and operation of this escutcheon, and also the general principle of cylinder escutcheons, are shown by the illustrations Figs. 388 and 389.

It will be seen that there are two barrels or cylinders, one rotating within the other, but eccentric with it. The lower cylinder is held from rotating by five sets of round pins, each set consisting of two pieces as shown in the section. When the key is drawn the pins are forced down into the lower cylinder so that it cannot be turned, but when the proper key is inserted in the lock all the pins are raised so that the joint in each set will just come on a line with the top of the lower cylinder, and the cylinder can then be rotated. A cam on the back of the rotating cylinder works the bolt in the lock.

It is evident that as the inner cylinder is exactly fitted to the bore in the shell, an almost imperceptible variation in the height to which-any one of the pins is raised will prevent the plug from turning, whence it follows that an immense number of locks can be made with such mechanism without duplication.

This arrangement of cylinders and pins is identical with that of the original Yale lock, the later improvements being in the shape of the key and keyhole. In the original Yale lock the keyhole was a narrow vertical slot, and it was possible for an expert lock-picker to open the lock by tilting a key or pieces of wire up and down in the keyhole until the pins were brought to the proper position for opening. To prevent this the corrugated key and keyhole was devised, and the new paracentric escutcheon is so constructed that it is impossible to insert any but the proper key in the keyhole or to use any picking instrument to operate the tumblers vertically, the shape of the keyhole shown on the face, Fig. 388, being continuous throughout the length of the lock.

Fig. 388.

Fig. 389.

An incidental advantage resulting from this change in the keyhole is that as it differs absolutely from every predecessor, no key heretofore made can enter one of these locks. The paracentric key is also a very difficult one to make, and the blanks can only be obtained from the manufacturers, hence the difficulty in duplicating a key,

As before staled, there are several other kinds of cylinder locks, a few of which somewhat resemble the old style Yale lock with corrugated keyhole. All of these locks offer greater security against picking or accidental interchange of keys than tumbler locks, and they are therefore considered as the best lock for front doors, office and store doors, drawers, lockers, and wherever special security is desired. A secondary advantage possessed by these locks, is the smallness and convenient size of the key.