Types Of Lamp Bases Explained | Match Codes To Sockets

Lamp bases are standardized connector ends — screw-in, twist-and-lock, pin, bayonet, and wedge — that let a bulb fit a socket and make electrical contact.

A bulb with a perfect color temperature and the right lumen output still sits useless in the box if its base doesn’t match the socket. The base carries the current, and for many lamps it also holds the whole bulb in place, so the connector end decides whether a fixture works at all. Most of the confusion comes from one habit: shopping by wattage or brightness while ignoring the short letter-and-number code stamped on the old bulb. This guide walks through the base families, the codes worth knowing, and the mismatch that trips up even careful buyers.

What Do The Letters And Numbers In A Base Code Mean?

Each code names a base family and a size, so the letter tells you the connector type and the number tells you its diameter or pin spacing. An E stands for Edison screw, and the number is roughly the thread diameter in millimeters — E26 is a 26 mm medium screw base and E12 is a 12 mm candelabra screw base. That “roughly” matters, though, because nominal numbers are not exact measurements.

North American screw bases in current use include E10 mini screw, E11 mini candelabra, E12 candelabra, E14 European small, E17 intermediate, E26 medium, and E39 mogul. Two twist-and-lock types show up often in modern residential fixtures: GU10 and GU24. Pin bases cover a similar range, with G4, GU4, GU5.3, GX5.3, GY5.3, G6.35, G9, and G12 among the common bi-pin options.

Two traps catch people here. First, an A19 is a bulb shape, not a base — shape and base are separate specifications. Second, the E-number families are not the same size class, so an E12 will never seat in an E17 socket.

Which Lamp Base Type Does Your Fixture Need?

Match the base code before anything else, then confirm voltage and physical clearance. Identifying the socket is the fastest route: check the socket itself, any labeling on the fixture, or the manufacturer’s documentation, and read the code off the old bulb while you’re at it. Fixtures sold in the US almost always take E26, while imported fixtures sometimes carry E27, so it’s worth verifying rather than assuming.

Voltage is the second filter. E26 is the standard North American screw base for 120 V lamps, and E27 is the international equivalent built for 230 V service. A bulb can have the correct base code and still fail if the fixture has limited room or a mismatched voltage requirement. For bayonet, wedge, GU10, and bi-pin bases, there is no cross-compatibility at all without a purpose-built adapter or the right fixture.

Are E26 And E27 Bulbs Interchangeable?

Not always — E26 and E27 share a thread diameter but differ in screw length, so a bulb can thread in partway and still fail to seat. Both are Edison screw bases, and when made to IEC standards both fall in the same thread-diameter range of 26.05 mm to 26.45 mm. The minimum screw length is where they part ways: 19.56 mm for E26 and 22.0 mm for E27.

That gap is why fixture clearance and insulator length matter. An E27 bulb can bottom out before its center contact reaches the socket in a shallow E26 fixture, and an E26 bulb in a deep E27 socket may sit too far in to make solid contact. Just as important, the two are rated for different mains voltages, so even a clean mechanical fit doesn’t make the swap electrically safe. On a US 120 V circuit, an E27 lamp marked for 230 V is the wrong bulb regardless of how well it threads.

If you’re replacing bulbs across several fixtures at once and want fewer trips to the store, this tested roundup of lamp bases worth buying narrows the options.

Where Do The Other Base Families Fit?

Bayonet, wedge, and pin families fill roles the screw base can’t, and none of them swap into a screw socket. Bayonet bases like BA15d and BA15s lock in with a push-and-twist motion, while wedge bases — including slide and subminiature wedge versions — push straight in with no rotation.

Fluorescent and compact fluorescent lamps use their own pin arrangements, which is why you can’t simply upgrade a fluorescent tube to LED without checking the base. Fluorescent pin bases include miniature bi-pin, recessed double contact, 4-pin, 2GX13, and axial types. Plug-in compact fluorescent bases run across the G23, G24d, G24q, GX24q, 2G7, and 2G11 families, among others.

For a plain answer on compatibility, the base must match the fixture socket and the fixture’s rated voltage, full stop. Adapters exist for a few combinations, but they add length that may not clear a tight fixture. When in doubt, the base code on the old bulb is the specification to beat.