The small hole near the end of the power (non-ground) blades of some NEMA plugs is used for convenience in manufacturing; if present, it must be of specified diameter and position. Small specialized padlocks are available to fit these holes, allowing "lockout" of hazardous equipment, by physically preventing insertion of locked plugs into a power receptacle. Since at least 1949, numerous receptacle devices have also been invented to use these holes to hold the prongs inside the receptacle slots, using a corresponding latch or locking mechanism.