National Electrical Manufacturers Association. Those responsible for the phrases "L6-20," "L21-30," "NEMA 5-15," etc. Here's a great .PDF chart, View attachment 2006_NEMA_Chart_Countermat_101107.pdf courtesy of Eaton. I suggest EVERYONE in the US print it out as big as possible and post it on the electrics workshop wall. NEMA also sets standards for electrical enclosures, and many other items electrical.