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Left-Hand Thread
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Most threaded fasteners are right-threaded, so you turn them clockwise to tighten them. Some parts which are used in conjunction with rotating shafts would tend to unscrew themselves if they threaded normally, so they are threaded in the opposite direction. This is called a "left thread," "left hand thread" or "reverse thread."
Left-threaded fasteners are turned counter-clockwise to tighten them.
Left threads are found on left pedals, some bottom-bracket parts, internal parts of freewheels, and sprockets that thread onto older freewheels from the left side.
Left-side pedals and right-side bottom bracket cups use left-hand threading because of the phenomenon called "precession." It is somewhat counterintuitive, because the direction of rotation caused by the precession is opposite the (much smaller) torque exerted by bearing friction.
See the Pedals entry in this Glossary , and Jobst Brandt's article on Left-Hand Threads on this site for a detailed explanation of precession.
Spoke nipples and the expander bolts that hold bar-con shifters in place often appear to have left threads, because they are approached from the opposite direction of most fasteners.
