The 120-volt power outlet is the standard electrical outlet in use in homes in North America. These outlets have been in use in their present form since the early 1950s.
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The modern 120-volt power outlet has three holes: two parallel rectangular slots—one connected to the "live" wire and one connected to the neutral wire—and a round hole for the ground wire. Most outlets are "duplex" receptacles, with two places to plug in electrical devices.
There are two common styles of 120-volt duplex receptacles. On "standard" receptacles, each outlet has its own round opening in the wall plate. On "decorator" receptacles, a single plastic rectangle holds both outlets.
All modern 120-volt outlets are designed to accommodate "polarized" plugs, on which the rectangular prong for the neutral wire is slightly larger. This is a safety measure designed to control the way an electrical device handles current.
120 Volts vs. 110 Volts
Any device that says it runs on 110 volts can be plugged into a 120-volt outlet. The "120 volt" label is just a nominal figure; the actual voltage could be anywhere in the range of 110-125 volts, and modern electrical devices are built to tolerate the fluctuations.
Some heavy appliances—such as electric clothes dryers—require 240 volts of power. Outlets that deliver 240 volts are configured differently, so devices for 120-volt outlets can't plug in to them.