Derailleur hangers, or mech hangers as they are otherwise known form the link between the bicycle frame and the rear derailleur.
The hanger is intended as a cheap replaceable part so that in the event of an accident or mechanical problem (such as a stick getting lodged in the rear wheel and pulling the derailleur off the frame), no significant damage comes to the frame or derailleur. Sometimes a shear bolt which is designed as a weak point will also be used.
Derailleur hangers are generally not used on steel frames, where any frame damage can often be repaired inexpensively due to the material being less likely to work harden during deformation.
