Cage nut or caged nut (also called a captive or clip nut) consists of a (usually square) 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.
The square-hole cage nut can be used wherever a square hole can be punched. An older alternative type of captive-nut uses a spring clip that holds the nut and slides on the edge of a thin sheet. While this type of cage nut can only position the nut a fixed distance from the edge of a thin plate, it works equally well with square and round holes