1. On a front derailleur of a bicycle , it is a pair of parallel plates that push the chain from side to side; on a rear derailleur , it is a set of plates in which pulleys are mounted to hold and guide the chain from cog to cog .
2. Any device for holding or securing something, e.g., a bottle cage on a bicycle .
3. When referring to bearings, it is the part which holds the balls or rollers in place. 4. When referring to a vehicle, it is the safety enclosure called a roll cage .