As I understand it, older appliances had a 3-wire cord with two hot wires and one neutral wire. An additional ground was added, making requiring a 4-prong plug.

As I understand it, older appliances had a 3-wire cord with two hot wires and one neutral wire. An additional ground was added, making requiring a 4-prong plug. We came across this when we replaced my mother's electric stove. When I bought my house, the (nimrod) flippers didn't hook up the stove, they just pushed it into place. Never having installed an electric stove, they didn't know you order the plug separately. As it turned out, the connection was so old that I had to special order a plug.