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A wafer tumbler lock is a type of lock that utilizes a set of flat wafers to prevent the lock from opening unless the correct key is inserted. This type of lock is similar to the pin tumbler lock and works on a similar principle. However, unlike the pin tumbler lock, the wafer is a single piece. The wafer tumbler lock is often incorrectly referred to as a disc tumbler lock, which uses an entirely differently mechanism.

Without a key in the lock, the wafers (red) are pushed down by springs. The wafers nestle into a groove in the lower part of the outer cylinder (green) preventing the plug (yellow) from rotating.

When the correct key is inserted, the wafers (red) are raised up out of the lower groove in the outer cylinder, but not so high that they enter the upper groove in that cylinder.

With the wafers unobstructed, the key, wafers, and plug (yellow) can rotate.

In a cylindrical wafer tumbler lock, a series of flat wafers holds a cylindrical plug in place. The wafers are fitted into vertical slots in the plug, and are spring-loaded, causing them to protrude into diametrically opposed wide grooves in the outer casing of the lock. As long as any of the wafers protrude into one of the wide grooves, rotation of the plug is blocked, as would be the case if there was no key, or if an improperly bitted key were inserted.

A rectangular hole is cut into the center of each wafer; the vertical position of the holes in the wafers vary, so a key must have notches corresponding to the height of the hole in each wafer, so that each wafer is pulled in to the point where the wafer edges are flush with the plug, clearing the way for the plug to rotate in order to open the lock. If any wafer is insufficiently raised, or raised too high, the wafer edge will be in one of the grooves, blocking rotation.

Wafer tumbler lock configurations vary with manufacturer. The most common is the single-bitted, five-wafer configuration most commonly found on desk and cabinet locks and some key switches. The illustrations represent this type.

Some wafer tumbler locks utilize a stack of closely-spaced wafers designed to fit a specific contour of a double-sided key and works on the principle of a carpenter's contour gauge.

Wafer tumbler locks can utilize single-bitted or double-bitted keys. Though wafer arrangements within the plug may vary, such as automotive locks, where the wafers are arranged in opposed sets, requiring a double-bitted key, the operating principle remains the same.