The TwistLock® connector was patented by the Harvey Hubbell company in 1938, (today, Welcome to Hubbell Wiring Device-Kellems). Because it is a locking connector, some prefer it to the stage pin connector. In the 1980s there was a (failed) movement to eliminate the stage pin connector and force all theaters, especially amateur and educational facilities, to convert.
Twistlock connectors come in a wide varieties of sizes, pin configurations, and voltages, based on a standardized system by NEMA.
See also Turn-Lock, for a competing but compatible product.
See also, NEMA.