We are sure you know there is a lot of information you need when shopping for light bulbs. Watts, lumens, kelvins, CRI, and even hours are all things you will need to know. The most important thing to know when buying a bulb might be the simplest of all, the base. Even if you get everything else just right, a bulb with the wrong base for your fixture will leave you in the dark every time.

If you are looking for bulbs in your home, chances are you will use Edison screw light bulbs, whether they are incandescent, compact fluorescent (CFL), or even newer LED bulbs. There are four common types of Edison light bulb bases, Medium, Mogul, Intermediate, and Candelabra. It’s easy to know which is which, as each will be labeled on the packaging with E (for Edison) and the diameter of the bulb base (in millimeters).

Medium (E26) base are the most common type of Edison screw base. Medium bases are used with standard A19 shape incandescent, CFL, and LED. The majority of table lamps and bathroom fixtures will take bulbs with medium base. Mogul (E39) base are used for high voltage metal halide and high pressure sodium bulbs. Intermediate (E17) base bulbs are uncommon, but are used in some appliances and in some specialty decorative bulbs. Candelabra (E12) base bulbs are used in many decorative fixtures, such as chandeliers.