["Followup-To:" header set to sci.electronics.design.]
Geoffrey S. Mendelson wrote:

There are several kinds depending upon the current rating and if 120
volts is used too. Before 1996, the ground pin was also used for neutral
for things like clothes dryers that had a 120 volt motors and timers and
240 volt heaters. I think that practice was stopped in the late 1970's.

In 1996, it became illegal. If your device has a mixture of 120 and 240
volt components, you need to have a four wire plug. I left the U.S. in
1996, so I've never seen them.

Just to add more aspects to this discussion. In Germany, practically all
houses have 400V three-phase electricity, which is three 230V phases 120°
degrees apart. So all the normal 230V outlets are just a single phase out of
those three plus neutral.

Big appliances in a fixed location like electric ovens and water heaters get
all three phases but are not required to use them in balance.

The nice thing about this is that if you want to set up a workshop in your
house, all you need is some extra fuses and cable and a couple those nice
big, red CEKON sockets.

robert