Are you thinking about 240V circuits which used to use 3-wire circuits but now use 4-wire? The US run split-phase power to homes with +120V and -120V hot legs and neutral connected to center tap of transformer. Each 120V circuit uses one of the hot legs.

For 240V, can use 3-wire circuits by getting 240V between the +120V and -120V legs. There are 3-wire grounded plugs (hot-hot-ground) and non-grounded (hot-hot-neutral). Modern 240V wiring uses 4 wires, hot-hot-neutral-ground, which allows making 120V for low-power electronics from one hot to neutral.