Left-hand thread definition
Most threads on bolts have right-hand thread meaning that turning the bolt clockwise inserts or tightens the bolt; counterclockwise loosens or removes the bolt. But in left-hand thread the reverse is true. Turning the bolt clockwise loosens or removes the bolt. Left-hand thread is used on applications where the normal turning motion of the object is such that there is a possibility that the object could back out and be removed. For instance, the left pedal of a bicycle turns around the crankarm in a counterclockwise direction. If it had right-hand thread, the pedal could fall off. Thus all left pedals have left hand thread so that the motion of the pedal helps to keep the pedal tight on the crankarm .