Our greenhouse heater wore out after a number of years so we bought a replacement. It's essentially the same heater except that the new one is a different color and the plug is a different configuration. The old one has one round pin and two flat blades, with the blades at an angle to each other and above the round pin. The new one also has one round pin and two flat blades but the blades are lined up with each other, so I have to replace the receptacle with one that matches the new plug. Not a major project, just a minor annoyance and expense.

I've seen other 220 volt plug configurations; some have an L-shaped blade instead of the round pin, others have four blades in a circular pattern which lock when turned, etc. Is there any practical reason for all these variations?

Paul