Most screws are tightened by clockwise rotation, which is called a right-hand thread. Screws with a left-hand thread are used in exceptional cases, such as where the screw will be subject to counterclockwise torque, which would tend to loosen a right-hand screw. For this reason, the left-side pedal of a bicycle has a left-hand thread.

https://en.wikipedia.org/wiki/Screw

Cosas para identificar a quien no ha reparado nunca una bicicleta (yo)