The disclosure relates to a derailleur for a bicycle, comprising a B-knuckle assembly which is designed for mounting on the bicycle frame, a P-knuckle assembly, a parallelogram assembly via which the B-knuckle assembly and the P-knuckle assembly are movably connected to one another, a cage assembly in which a chain guide pulley and a chain tension pulley are rotatably held, where the cage assembly is mounted on the P-knuckle assembly so as to be rotatable about a P axis, and an rotation angle detection device which is designed to detect and/or display an angle of rotation between the P-knuckle assembly and the cage assembly.