The castles, churches and convents of the middle ages had their often highly ornamental locks and their warders to guard and open them. Later, locks were invented with complex wards. These are carved pieces of metal in the lock which fit into clefts or grooves in the key and prevent the lock from being opened except by its own proper key. As early as 1650 the Dutch had invented the Letter lock, the progenitor of the modern permutation lock, consisting of a lock the bolt of which is surrounded by several rings on which were cut the letters of the alphabet, which by a prearrangement on the part of the owner were made to spell a certain word or number of words before the lock could be opened. Carew, in verses written in 1621, refers to one of these locks as follows:-- “As doth a lock that goes with letters; for, till every one be known, The lock’s as fast as though you had found none.” The art had also advanced in the eighteenth century to the use of _tumblers_ in locks, the lever or latch or plate which falls into a notch of the bolt and prevents it from being shot until it has been raised or released by the action of the key. Barron in England in 1778 obtained a patent for such a lock. Joseph Bramah, who has before been referred to in connection with the hydraulic press he invented, also in 1784 invented and patented in England a lock which obtained a world-wide reputation and a century’s extensive use. It was the first, or among the first of locks which troubled modern burglars’ picks. Its leading features were a key with longitudinal slots, a barrel enclosing a spring, plates, called sliders, notched unequally and resting against the spring, a plate with a central perforation and slits leading therefrom to engage the notches of the slides simultaneously and allow the frame to be turned by the key so as to actuate the bolt. Chubb and Hobbs of England made important improvements in tumbler locks, which for a long time were regarded as unpickable. Most important advances have been made during the century in _Combination_ or _Permutation Locks_ and _Time Locks_. For a long time permutation or combination locks consisted of modifications of one general principle, and that was the Dutch letter lock already referred to, or the wheel lock, composed of a series of disks with letters around their edges. The interior arrangement is such as to prevent the bolt being shot until a series of letters were in line, forming a combination known only to the operator. Time locks are constructed on the principle of clockwork, so that they cannot be opened even with the proper key until a regulated interval of time has elapsed.