What is a 3-way light bulb?

A three-way light bulb has two different filaments inside, each with different wattage for the low and medium light settings, which combine for the bright setting. For instance, a 30-watt filament for low and a 70-watt filament for medium combine for a 100-watt light.

Compact fluorescent lamps, also known as CFLs, have two different wattage filaments similar to traditional bulbs. They differ in that CFLs rarely have two separate fluorescent tubes inside each housing. Instead, CFLs are controlled by internal voltage regulation or with a dimmer. Both types of bulbs use a three-way switch in the lamp to turn on each level of light.