Cage nuts are used almost exclusively in rack angles with square mounting holes. They consist of a square nut wrapped in a spring metal cage that is slightly larger than the nut. This spring steel has two flanges that when pinched together allow the component to fit into the square hole on 19″ racks. The nut is typically loose in the cage to allow for adjustments when installing the screw.
The three most common sizes of cage nuts are 10-32, 12-24 and M6. The size needed varies by the type of equipment being installed in the rack. Best practice is to also purchase the corresponding screw to go with the cage nut.