Some people who have responded are a little off track about residential single phase electric service so I thought I would try to resolve any confusion. As for my credentials, I am an electrical engineer and worked over 40 years for the local electric utility.
Most residential homes receive single phase, 120/240 volt service from their electric utility. Single phase means there is only one phase on that service. The transformer serving the house has two coils inside. One, the primary coil, is connected to the high voltage side with one end connected to the high voltage wire (for example, 7960 volts at the utility I worked for) and the other end connected to ground. This means there is 7960 volts across the primary coil. The other coil, the secondary coil, has three connections. The middle of the coil is connected to ground and is also connected to the neutral wire going to the house. The two outside ends of the coil are connected to the two "hot" or energized wires going to the house. The turns of wire in the secondary coil are designed to produce 120 volts in each HALF of the coil. Therefore one measures 120 volts from each hot leg to neutral or ground. The measurement from one hot wire to the other hot wire is 240 volts because half of the secondary coil adds voltage to the other half of the secondary coil (120v + 120v). Another way to look at it is there is 240 volts across the entire length of the secondary coil and 120 volts across half of it. Electric voltage on both hot wires have the same sine wave so they are the same phase.
Hopefully this helps.
------ Jim
Thanks for taking the time to explain this clearly.
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You probably don't need it, but I'll attach it anyway. It's what I used when I wired a 50A motorhome receptacle.
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Brad and Pat, the Dad and Mom-Dawgs with Kiddo and Dinah-Mite, the Dawter-Dawgs full timin' in our 2002 HR Neptune DP with 2017 Grand Caravan toad