What is a 3-way dimmer switch?

A three-way dimmer switch is a light switch that works in conjunction with another light switch to operate a common lighting element. Either one or both of the light switches also control the intensity of the light output.

A common application for a three-way light switch is controlling a hallway light at the top of the stairs from both the upper and lower floors. Dimmer functionality is not common in the above scenario; however, three-way dimmer switches are very common in large areas such as a dining room or other area with a single lighting fixture that is installed centrally, usually from the ceiling.