What is a key bit?

A key bit (Dutch: baard) is the flat, protruding part of a key that engages with the lock mechanism to open or close it. In masonry, the term also refers to mortar squeeze-out that protrudes between brick joints.

Application

In locksmithing, the bit is the shaped, flat section at the end of a traditional key. When the key is turned, the bit moves the levers or tumblers inside the lock, allowing it to open or close. The notches and shape of the bit are unique to each lock, determining which key fits. On older, classic keys, the bit is clearly visible as a distinctive protruding plate at the end of the key shaft.

In masonry, the term refers to the mortar that squeezes out between bricks during the bricklaying process. These mortar remnants become visible on the inner side of the wall. In cavity walls, excessive mortar squeeze-out can be problematic, as mortar debris falling into the cavity can create thermal bridges. It can also obstruct drainage within the cavity. A skilled bricklayer ensures that mortar squeeze-out is kept to a minimum by using the correct amount of mortar.

During restoration work on historic buildings, both meanings of the term are regularly encountered: old locks with distinctive key bits and brickwork where mortar protrusions have been deliberately left visible as a stylistic feature.

Related terms

• Key shaft
• Strike plate
• Mortar
• Cavity wall
• Pointing
• Thermal bridge

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