What is a three-way switch and how is it different?

A three-way switch controls one light from two locations — the top and bottom of a stairwell, or both ends of a long hallway. Unlike a single-pole switch, it has no fixed on and off; each switch simply toggles the circuit, so either can turn the light on or off regardless of the other's position. It has three terminals plus ground: one common (the odd-colored screw) and two travelers. You always install them in pairs.

Which screw is the common on a three-way switch?

The common terminal is the odd one out — usually a dark or black screw, often labeled COMMON on the back, while the other two traveler screws are brass or a lighter color. On the incoming-power switch, the common takes the hot from the panel; on the outgoing switch, the common feeds the light. Getting the common right is the whole job — the two travelers are interchangeable, but the common is not.

How do I identify the wires when replacing a three-way switch?

Before disconnecting anything, label the wire attached to the common (dark) screw with a piece of tape — that is the one that must return to the common on the new switch. The other two are travelers and can go on either traveler terminal. If you have already disconnected without labeling, you may need to test which wire is the hot common with the power on and a tester, which is fiddly — so always label first.

Why does my three-way switch only work sometimes?

The classic symptom of a miswired three-way is that the light works from one switch only when the other is in a certain position. That means a traveler and the common got swapped. Kill the power and confirm the common wire lands on the common (dark) screw at both switches, and the two travelers connect to the traveler terminals. Once the commons are correct, both switches control the light from any position.

Can I replace one three-way switch with a dimmer?

Yes, but you need a three-way dimmer, and typically only one of the two switches can be the dimmer (the other stays a standard three-way switch, or you buy a matched companion dimmer set). Wire the dimmer's common lead to the common wire and its two traveler leads to the travelers. Installing a single-pole dimmer in a three-way circuit is the most common failed job — it will not work correctly.

How to Wire a Three-Way Switch: A Careful DIY Guide

Written & researched by Derek Holt Home & Appliances Editor

Updated August 2026

Over-the-shoulder view of a person switching, in warm late-afternoon light

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Wiring a three-way switch is an intermediate DIY job — plan on 20 to 40 minutes per switch, a few dollars for the switch, and no permit for a like-for-like replacement. A three-way lets two switches control one light, using a common terminal and two travelers. The entire trick is identifying and preserving the common (the odd, dark screw). Cut the breaker, verify the wires are dead, and label the common wire before you disconnect anything. Anything at the service panel is a licensed electrician's job.

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Tools and materials

Non-contact voltage tester, screwdrivers, and wire strippers.
Electrical tape or wire labels — labeling the common is the most important step.
Two three-way switches (replace both together for consistency), or a three-way dimmer plus a companion — see our best dimmer switch picks for compatible three-way models.
A phone camera to photograph the existing wiring at each box.

Overhead view of a person switching, in soft overcast light

Step 1: Understand the layout

A three-way circuit has two switches and one light. Power enters at one switch's common, two travelers run between the switches, and the second switch's common feeds the light. The neutral runs straight through to the light and does not connect to the switches (except to power a smart switch). Knowing this map makes the wiring obvious rather than mysterious.

Step 2: Cut the power and verify dead

Switch off the breaker for the circuit and confirm both switch boxes are dead with the voltage tester before touching wires. Three-way circuits can have voltage present on travelers depending on switch position, so verify at both boxes.

Three-quarter view of a person switching, in soft overcast light

Step 3: Label the common before disconnecting

At the first switch, find the wire on the common (dark or COMMON-labeled) screw and wrap a piece of tape around it. The other two wires are travelers — leave them unmarked. This one label is what keeps the job simple; without it you are guessing which of three wires is the common.

Step 4: Connect the new switch

Land the taped common wire on the new switch's common (dark) screw. Connect the two travelers to the two traveler (brass) terminals — order does not matter between them. Connect the bare or green ground to the green grounding screw. Hook each wire clockwise around its screw and tighten firmly, or use the switch's terminals per its instructions. Repeat at the second switch, again landing its common on the common screw.

Wide view of a person switching, in bright midday sun

Step 5: Mount, power, and test

Fold the wires into the boxes, mount both switches, and add cover plates. Restore power and test the true three-way behavior: the light should turn on and off from either switch, in any combination of positions. If it only works from one switch when the other is set a certain way, a traveler and common got swapped — kill power and recheck the common connections.

Common mistakes

Not labeling the common. The single biggest cause of a miswired three-way. Tape it first.
Swapping a traveler onto the common screw. Produces the works-only-sometimes symptom.
Installing a single-pole switch or dimmer. A three-way circuit needs three-way devices.
Forgetting the ground. Land it on the green screw at both boxes.
Not verifying dead at both boxes. Travelers can carry voltage.

When to call an electrician

Bring in a licensed electrician if the wiring does not match the three-way pattern (extra cables, unclear commons), if you find aluminum wiring, if a box has no ground, or if the work reaches the panel — permit-and-pro territory, jurisdiction-specific, never DIY at the service panel. Replacing three-way switches on sound existing wiring, with the common labeled, is a homeowner job.

How to Wire a Three-Way Switch