A pin-tumbler lock invented by Linus Yale Jr
(1821-68) in 1865 and still widely used. It
consists of a cylinder which can rotate
inside a housing to turn the bolt. In the
cylinder is a row of holes, which match
another row in the housing. When the lock is
locked, two pins, held in each of the holes
by a spring, prevent the cylinder turning.
When the correct key is inserted, the lower
pins are raised so that they reach the edge
of the cylinder and allow it to turn.