The duplex receptacle was, until 1960, the most common type of household outlet in use.  It accepts a two-prong plug, the type most often found on lamps and small appliances.  In 1960, the NEC and the MPS required that all receptacles be of the grounding type, designed to also accept a three-prong plug.  Many small appliances are wired with a third ground wire that is attached to the frame or metal housing of the appliance.  The third slot in a grounded outlet is connected to a water pipe or other grounding metal.  Grounding of an appliance using a three–prong plug and receptacle reduces its shock hazard.

Special waterproof receptacles with caps are available for outside use, clock outlets, TV outlets, locking outlets and a variety of other special purpose outlets. The receptacle for a 220-volt to 240 volt line is designed to accept only special plugs.  A standard two-prong or three-prong plug cannot be plugged into it.  It also is designed to accept only plugs for appliances using only the exact number of amperes that it will supply.

Outlets should be located conveniently throughout the house.  The MPS requires that three-prong duplex grounding outlets should be installed in all habitable rooms so that no point along the floor line is more that 6 ft. from an outlet.  It also requires that an additional outlet between all doors and between doors and a fireplace should be supplied (unless the wall is too small for a piece of furniture); that in rooms without permanent light fixtures, at least three outlets should be provided regardless of the room size; that two outlets should be installed in the kitchen over the counters; and that an outlet should be installed next to the mirror in the bathroom.