Here's a good article from Wikipedia on the matter:
http://en.wikipedia.org/wiki/NEMA_connector#Non-locking_connectors
http://en.wikipedia.org/wiki/NEMA_connector#Non-locking_connectors
NEMA non-locking connectors all use blades of various flat and folded shapes (except for the round pins used on grounding connectors) and the plugs can be detached from the receptacles by pulling back on the plug body. The connector families have been designed so that grounding connectors for 120 V and 208/240 V cannot be accidentally intermated.
NEMA wall receptacles can be found installed in any orientation. Neither NEMA or the U.S. National Electrical Code nor the Canadian Electrical Code specify a preferred orientation, and different orientations may be found in the same building. However, the orientation with the ground pin on the bottom is the most common, which places the neutral pin on the upper left and the hot pin on the upper right. All descriptions below assume this orientation.
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It's job specs or personal preferances....