Not all sockets are created equal. They come in all sorts of shapes, sizes, and materials. Because of this, it’s easy to get confused about which type you need, but figuring it out doesn’t have to be a chore. With a few exceptions like axial or festoon socket fixtures, all sockets fall into four main groups: screw, pins, wedges, and bayonets.
Screw Sockets
Named after Thomas Edison, the Edison screw socket is the most common and easily recognizable socket type. These sockets have a designation code that starts with an “E” for “Edison” and a number which represents the diameter in millimeters of the light bulb base that fits into it. For example, an E26 socket is designed for use with an E26-based light bulb. E26 and E27 bases are often interchangeable, just be sure the bulb runs on the correct voltage and wattage. Commonly used screw sockets include:
| Designation | Name | Diameter of Base |
|---|---|---|
| E39 | Mogul | 39mm |
| E27 | European Medium | 27mm |
| E26 | Medium | 26mm |
| E17 | Intermediate | 17mm |
| E14 | European Intermediate | 14mm |
| E12 | Candelabra | 12mm |
| E11 | European Candelabra | 11mm |
Pin Sockets
Pin based sockets are used primarily with linear fluorescent tubes, plug-in CFL lamps, some mini indicator lamps, HID lamps, MR16s, and the LED versions of each. Pin base bulbs have between one and four pins sticking out from the bottom that connects the bulb to the electrical current. The designations of pin sockets start with a letter, different for each type of base but often a “G,” followed a number representing the distance between the pins in millimeters. Linear fluorescent and LED tube lights are pushed into their tombstone sockets then rotated into place. Similarly, GU24 base bulbs twist and lock into place.
Most pin bases and sockets are fairly straight forward, but a few can be a bit more complicated. 4-pin plug-in CFL bulbs follow the same designation as other pin base with a “q” following the pin spacing number, which stands for quadruple, then either a 1, 2, or 3. This final number indicates the base’s dowel position. You may always notice some CFL bases have an “X” following the “G” in their designation. GX bases are slightly shorter than bases without the “X” in the name. For example, a G24 base is taller than a GX24 base. Plug-in CFL sockets are designed to only work with one style of pin base to ensure the correct light bulb is used to match the ballast in the fixture. 2-pin sockets only work with magnetic ballasts while 4-pin sockets are limited to electronic ballasts. These sockets hold the bulbs in place using friction.
Wedge Sockets
Wedge sockets and bases are very distinct because unlike more traditional sockets and bases, the bases don’t have a metal cap or pins. Instead, the glass of the bulb tapers down to a point to seal it. Two wires extend from the bottom of the base and act as contact points.
Bayonet Sockets
Fairly uncommon in household lighting in the United States, bayonet sockets are named after the bayonets used on rifles because they have a similar connection method. The light bulbs that are used in bayonet sockets are pushed in then turned slightly to lock into place. The bulb’s base has two pins on either side while the socket has two corresponding L-shaped slots and a small spring to keep the two locked together. The locking mechanism makes bayonet sockets a good choice for applications where vibration may loosen Edison screw bulbs, such as automotive lighting. These sockets have either one or two contact points at the bottom depending on which type of bayonet base bulb is used. Bayonet bases have the designation of “BA” followed by a number that represents the base’s diameter in millimeters then either an “s” or “d” to signify if it is a single or double contact.
