Mushroom joint

A mushroom joint is a type of swivel joint. It is based around a peg or pin with a wide ("mushroomed") top, over which another piece can be fitted. The widened portion is what holds the swiveling piece in place. Mushroom joints are typically found in built-in accessories, such as Battle Blades Optimus Prime's swords or the smaller guns on Bulkhead's small roof-mounted guns. They are designed primarily to be an easy way to attach permanent parts while being detachable for safety reasons. In that regard, they serve much the same purpose as ball joints, but in cases where the part in question should only rotate in one direction.