Bakcou Rear Derailleur Hangers 

This part is what attaches your derailleur (the component that enables gear shifting) to your bike frame. Designed to fail in the event of a crash, the derailleur hanger protects both your bike frame and derailleur from damage. It’s wise to keep a spare derailleur hanger on hand.

Note: The derailleur hanger is specific to your model of bike