The US uses split phase, so 120VAC is 120 volts RMS, 170 volts peak or 340 volts peak-to-peak, centered around neutral/ground, and 240VAC is 240 volts RMS, but made from two 120VAC outputs which are 180 degrees out of phase.
At the transformer, the secondary is 240VAC with a center tap attached to neutral, and neutral is attached to Earth ground at the distribution panel. Inside the building, the 120VAC sockets are distributed between the two 180 degree phases, which means in theory 240VAC can be regained by using the hot lead from two different sockets, not that I would ever have done that. Also, I have never wired a duplex output with one phase on one outlet and the other phase on the other outlet. That is my story and I am sticking to it.