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Keyed Washer
From WikiPedalia
A keyed washer is a washer with a special-shaped hole that fits over a special shaft. It can slide, but not turn. The usual set up is to have a groove in the axle or shaft. A washer's hole will have a small tang that fits into the groove. This is almost always used in headset and pedal bearings. It used to be common in hub bearings as well, but has fallen out of favor for that application.
Another type of keyed washer fits a shaft that is round but has one (or more) flat side(s). French forks usually use this system, with a simple filed flat on the back of the steerer.
