Cage nuts consist of a nut in a spring steel cage which wraps around the nut. The cage has two wings that when compressed allow the cage to be inserted into the square holes, for example, in the mounting rails of equipment racks. When the wings are released, they hold the nut in position behind the hole.
Cage nuts conforming to this description were patented in 1952 and 1953.This design requires insertion tools to install the cage nut into the hole. Newer designs featuring a squeeze-and-release tab allow for tool-less installation.