Three-Prong Plug
The standard three-prong plug can only be inserted one way to ensure the proper function of the three-wire system.
Source
Boundless vets and curates high-quality, openly licensed content from around the Internet. This particular resource used the following sources:
"OpenStax College, College Physics. October 30, 2012."
http://cnx.org/content/m42416/latest/?collection=col11406/1.7
OpenStax CNX
CC BY 3.0.