Tom's dad is correct. Years ago the voltage was 110v on each leg.
Then the electric Co's. upped it to 115, then to 117, then to 120.
Put the two legs together from the step down transformer (which is located on the top of the telephone pole) with a resistance between them and you have double the voltage! Neat huh ?

Why did the electric Co's up the voltage? Well there was a reason. I used to know but can't remember now. But I will take two guesses.
1) To make the light bulbs burn out faster for the economy's sake or
2) Voltage is pressure (or force). The higher the voltage- the further you can supply it. That is why the main supply is somewhere between 10 - 20 KV (Kilo volts) Otherwise said 10 - 20 thousand volts. When this flow of electrons reaches where they want it to go , it is stepped down to 120 - 240v via a stepdown transformer after which, it enters your home. You have 3 wires entering your home. Two of which are 120v legs and the third is , what they call now , the nutral (years ago we called it the ground) Soooo, I think that they kept upping the voltage , on the main trunk , to get it out further without having to change all the step-down transformers.

This is the reason DC , direct currant (Tom Edison's idea) , didn't work.
You need AC to use transformers (wheather up or down) and 120v ac or dc will not travel very far. Hence, Mr Westinghouse had a better idea. Wack everybody with kilovolts and hide it with step down transformers.