Most US residential housing is fed off a 240V center tapped transformer. Center tap is connected to ground and neutral at the breaker panel. That gives you two 120V phases to neutral. And 240V from phase to phase.
In the breaker panels the phase alternates as you go down the rows of breakers. So you can install a dual breaker for 240V. Typically only large appliances like stoves, water heater, driers, and air conditioners are 240V. Everything else is 120V.
