A plug constructed to allow insertion into a receptacle only one way because one blade is wider than the other.
Plug with an equipment prong longer that the two blades, one of which is wider than the other.
Electrical plugs designed so that they can only be plugged into a socket in one direction. This is accomplished by having one blade of a two prong plug larger than the other or by the addition of a third ground prong.
Electric plug that guarantees that the hot side of a receptacle is connected to the hot side of an appliance because it can be inserted into the receptacle only one way. When the appliance switch is off, the power to the appliance stops at the switch.