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Anti-rotation Washer
From WikiPedalia
Internal gear hubs use the axle as a fulcrum for the gearing, tending to twist the axle, and in opposite directions in some gears. Such hubs need some method of keeping the axle from rotating in the frame. The most common approach is to use special washers with tabs that fit into the dropout's slot and keep the axle from turning.
The 14 speed Rohloff Speedhub has such a wide gear range that simple anti-rotation washers aren't sufficient, so it uses either a special dropout or a reaction arm connecting to a disc brake braze-on.
