Bulb Bases and Filaments

The base is the half of the puzzle most people forget — a bulb that's the right shape but the wrong base won't fit your fixture. For screw bases, the number after the E is the base diameter in millimeters (an E26 "medium" base is 26mm across — the US standard). The chart below covers screw, twist & lock, specialty, bi-pin, fluorescent, and plug-in bases, plus filament types.

Base Types Quick Reference

Screw Bases: E10 Mini Screw, E11 Mini Candelabra, E12 Candelabra, E14 European, E17 Intermediate, E26 Medium (US Standard), E27 Medium (European), E39 Mogul, E40 Mogul, EX39 Extended Mogul.

Twist & Lock Bases: GU10 (10mm), GU24 (24mm) — push-and-twist bases common on halogen spots and California-compliant fixtures.

Specialty Bases: Medium Side Prong, End Prong, M-P Screw Terminal, S14s, Rigid Loop (Rigid Loop Cap), Festoon, Wedge, R7s Recessed Single Contact, BA15d Double Contact Bayonet, BA15s Single Contact Bayonet, SC Single Contact.

Bi-Pin Bases: G4 (4mm), GU4 (4mm), GU5.3 (5.3mm), GY6.35 (6.35mm), GU8 (8mm), GY8 (8mm), GY8.6 (8.6mm), G9 (9mm), G12 (12mm) — the number is the pin spacing in millimeters.

Fluorescent Pin Bases: Mini Bi-Pin, Recessed D.C. (double contact), G10q 4-Pin, Medium Bi-Pin, Single Pin, 2GX13, Axial.

Compact Fluorescent Plug-In Lamp Bases: G24q-1, G24q-2, G24q-3, GX24q-2, GX24q-3, GX24q-4, GX24q-5, G-23, G23-2, GX23, GX23-2, G24d-1, G24d-2, G24d-3, GX32d-2, GX32d-3, 2G7, 2GX7, 2G11, GX10q-4.

Filament Types

Flicker, C-2V / CC-2V, C2R, C-5, C5V, C-6 / CC-6, CC6A, C-7A / CC-7A, CC-8 Double Ended, C-8 / CC-8, C-9 / CC-9, C-11 / CC-11, C-11V, C-13, C-13D, C-17, C-22.

Note: C (coiled) — filament wire is wound into a helical coil or is deeply fluted. CC (coiled coil) — filament wire is wound into a helical coil, and this coiled wire is wound again into a helical coil.

Pair this chart with our Bulb Shapes and Sizes guide to identify any lamp. Can't match your base? Send us a photo — we've been identifying odd bulbs since 1992.