A cage nut is used on a rack that has square mounting holes. You would use a cage nut when you need to mount a product in your rack that requires a threaded hole. A cage nut is nothing more than a square nut that has threaded holes wrapped in a piece of spring steel designed like a cage. This spring steel has two flanges that when pressed together allow the cage nut to be inserted into the square hole and simply clip in. When this is done, you now have a square hole that is threaded. The nut is typically loose in the cage to allow for adjustments when installing the screw.