Light sockets come in a variety of sizes, shapes, and materials. With so many options, it’s easy to get confused when you are trying to figure out which socket you have. Most sockets aside from a few exceptions, such as festoon or axial, fall into one of these four groups: screw, pins, wedges, and bayonets.

Screw Sockets

The Edison screw socket, named after Thomas Edison, is the most common and recognizable socket type. These sockets use a designation code of “E” for “Edison” and a number that represents the diameter (in millimeters) of the light bulb base that it fits. For example, an E12-based light bulb has a diameter of 12 millimeters and should be used with an E12 socket. E26 and E27 bases can be interchanged since a single millimeter doesn’t make a difference, but make sure that the bulb is using the correct voltage and wattage. The most common screw sockets are:

  • Mogul (E39)
  • European Medium (E27)
  • Medium (E26)
  • Intermediate (E17)
  • European (E14)
  • Candelabra (E12)
  • European Candelabra (E11)

Edison sockets have metal threaded contacts with housings made of plastic, porcelain, or metal. Porcelain and plastic housings typically come in either black or white. Metal screw sockets can be found in a variety of bronze, brass, or nickel finishes with different color variations depending on the manufacturer.Choosing which material to use is purely aesthetic, butplastic sockets may not be able to handle the hotter temperatures some light sources produce.

Pin Sockets

Pin based sockets are used with bulbs that have one to four pins coming out from the bottom of the base to connect to the electrical current.These bulbs include linear fluorescent tubes, plug-in CFL lamps, HID lamps, MR16s, some mini indicator lamps, and the LED versions of each. Sockets are designated with a letter, often a “G”, followed a number representing the length (in millimeters) between the pins.LED tube lightsand linear fluorescents are pushed into sockets and then rotated into place. GU24 base bulbs also twist and lock in a similar fashion.

A few pin bases and sockets can be a little more complicated than others. Some 4-pin plug-in CFL bulbs may have a “q” after the pin spacing number, meaning quadruple, followed by a 1, 2, or 3 to indicate the dowel position. Other CFL bases with an “X” following the “G” will be shorter than a base without the “X”. For instance, a GX24 base is shorter than a G24 base. In order to ensure that the correct ballast and light bulbs are matched, plug-in CFL sockets are designed to work with only one style of pin base. 4-pin sockets use electronic ballasts while 2-pin sockets use magnetic ballasts. Both sockets use friction to hold the bulbs in place.

Wedge Sockets

Featuring a very distinct look, wedge sockets and bases don’t have metal caps or pins. The base of the bulb tapers down to a point with two wires extending out. The bulb simply pushes into the socket to seal it, with the wires acting as contact points.

Bayonet Sockets

Named after rifle bayonets due to their similar connection method, bayonet sockets are not normally used for household lighting in the United States but are more commonly found in cars and SUVs. The base of bayonet bulbs have one or two pins on either side that corresponds with L-shaped slots on the socket. The bulb is pushed into the socket and turnedto lock in place, and a small spring at the bottom helps keep the bulb from moving. This makes bayonet sockets ideal for applications like automotive lighting where vibration may loosen other types of bulbs.The designation for these sockets starts with “BA” followed by a number representing the base’s diameter (in millimeters), and then either “s” or “d” is used to show if the socket is a single or double contact.

You should now be able to easily identify your light sockets now that you are familiar with the basic types.