Three-Way Switches

Control one light from two locations. The 3-way switch is the first circuit where a switch is not just on or off — it picks a path.

The idea: one light, two locations

A 3-way switch lets you control a single light from two places — top and bottom of a staircase, two doors of a room. The trick is that neither switch is simply "on" or "off." Instead, each one is a single-pole, double-throw (SPDT) switch: a common terminal that connects to one of two traveler wires.

How it works

Power feeds the common of the first switch. Two travelers run between the two switches. The light connects to the common of the second switch. The light is on whenever the two switches route a complete path through the same traveler — and off when they don't. Flip either switch and you change which traveler it uses, which makes or breaks the path. That's why either location can toggle the light regardless of what the other switch is doing.

Reading it

Don't think "on/off" — think "which traveler." At any moment each switch is sitting on traveler 1 or traveler 2. Same traveler on both ends = complete path = light on. Different travelers = broken path = light off. Flipping either switch swaps its traveler and flips the result.

What to take away

Two 3-way (SPDT) switches and two travelers give you control of one load from two locations. Each switch routes a common to one of two travelers; the light is on when both ends share a traveler.