Mike,
Just to add a few more words of explanation from an outsiders persepctive for you.
The pins are the same shape and size on all 120 and 240V plugs rated for 15 and 20A. It is the pin configuration which differs. If you look at the chart provided in Ron's link, the normal 120V 15A type is NEMA 5-15 which has parallel pins. The 240V 15A version is type 6-15, in which the pins are turned through 90 degrees to be in line.
The 20A versions of each have one pin set at 90 degrees to the other (types 5-20 and 6-20). The 20A receptacles for each have one slot T-shaped. In other words, a 120V 15A receptacle will accept only the corresponding 15A plug, but a 120V 20A outlet will accept either a 15 or a 20A plug. The same with the 240V versions, but of course the arrangement of which pin is turned through 90 degreees is different to prevent 120 plugs entering 240 outlets and vice versa.
The other types common in residential wiring are 10-30 or 14-30 for dryers, and 10-50 or 14-50 for ranges.
By the way, the term duplex refers to a device with two receptacles on the same yoke -- What in other countries might be called a double or twin socket.
[This message has been edited by pauluk (edited 05-13-2003).]
