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A warded lock (also called a ward lock) is a type of lock that uses a set of obstructions, or wards, to prevent the lock from opening unless the correct key is inserted. The correct key has notches or slots corresponding to the obstructions in the lock, allowing it to rotate freely inside the lock. Large warded locks are still in use today in the UK and Ireland for internal and external doors though mainly in 'heritage' sites such as ancient monuments and churches to preserve original features, primary security being provided by other means. Warded locks in America are commonly used in inexpensive padlocks, cabinet locks, and other low-security applications, since a well-designed skeleton key can successfully open a wide variety of warded locks.
The warded lock is one of the most ancient lock designs still in modern use. During the Middle Ages they were used prolifically on monasteries where, because money and time were not issues, their complexity grew. All had the same inherent problem: by removing most parts of the key, except the bare minimum to operate the bolt, and so producing a skeleton key, the wards could be bypassed.
In the most basic warded lock, a set of obstructions, often consisting of concentric plates protruding outwards, blocks the rotation of a key not designed for that lock. Warded locks may have one simple ward, or many intricate wards with bends and complex protrusions; the principle remains the same. Unless the notches or slots in the key correspond to the wards in the lock, the key will strike an obstruction and will not turn.
Additionally, a series of grooves on either side of the key's blade limit the type of lock the key can slide into. As the key slides into the lock through the keyway, the wards align with the grooves in the key's profile to allow or deny entry into the lock cylinder. Although this is not commonly recognized when discussing warded locks, it is more applicable to current locksmithing applications such as pin-tumbler or wafer-tumbler locks.
In double sided locks the centre of the key shaft is solid and protrudes past the end of the bit which slots into a hole on the opposite side of the lock. Double sided ward locks nearly always have perfectly symmetrical bits. For single sided locks a cylindrical post is typically located in the center of the lock. Its purpose is to provide a point of leverage for rotating the key, and to help correctly align the key with the wards. The key has a corresponding hole which fits over the post.
When the correct key is inserted, it will clear the wards and rotate about the center post. The key may then strike a lever, activating a latch or sliding bolt, or it may itself push against the latch or bolt. In a double acting lever lock, the key may additionally push against a spring-loaded lever which holds the sliding bolt in place.