[Boatanchors] AC plug & receptacle question

[Yong Surk Lee]

 US uses 117volt AC household current. 
Most AC plugs have two pins, one wide and the other narrow. 
Also AC receptacles have two holes the same way. 
So the plug always fits one way. 
Why is this necessary? 
Some of the US AC plug has the third pin which I believe is ground. 
I have a US made AC cord that have 3 pins and 3 wires connected to it, 
white, black and green. I know the green is ground. The white and black wires 
are interchangeable? 
de HL1FB/WW6N