Collection: Bulbs by base type

What is a car bulb base?

The base is the metal part at the bottom of the bulb that allows it to be secured in the vehicle's socket and establish electrical contact. In automobiles, bases are standardized by European regulation R37, and each type of base corresponds to a specific function (low beam, turn signal, side light, brake light, etc.).

Unlike household bulbs (E27, E14, B22, G9 — all screw-in or bayonet for wall sockets), automotive bases are designed to withstand the shocks, vibrations, and high temperatures encountered in a vehicle.

The most common automotive bases

BA15 / BAY15 / BAZ15 / BAU15 Family (15 mm diameter)

This family includes bayonet bulbs with a 15 mm diameter, among the most common for signal lights.

• BA15s — 2 symmetrical pins, single filament. Used for P21W (turn signals, fog light) and R5W (side lights).
• BAY15D — Offset pins, double filament. Used for P21/5W (tail light + combined brake light).
• BAU15s — Pins offset at 30°. Used for PY21W orange (turn signals).
• BAZ15D — Pins offset in 2 directions. Used for P21/4W (tail light + brake light).
• BAW15s — Used for PR21W red (fog light).
• BA15D — Symmetrical pins, double filament. Mainly old cars.

👉 For technical details, consult our article BA15 Bases: the differences.

BA9 / BAY9 / BAX9 Family (9 mm diameter)

• BA9s — Symmetrical pins. Used for T4W (interior lighting, license plate).
• BAY9s — Offset pins. Used for H21W (rear fog light, turn signals).
• BAX9s — Pins offset at 9°. Used for H6W (side lights).
• BAW9s — Used for HY21W orange.

Pinch Bases (low beam / high beam)

• P14.5s — Single pin base with flat tab. Used for the H1 bulb (low beam, high beam, fog light).
• PX26d — Rectangular base with 2 terminals. Used for the H7 bulb (low beam / high beam).
• PK22s — Used for the H3 bulb (front fog light) with its flexible wire.
• P43t — Base with 3 pins. Used for the H4 bulb with double filament (low beam + high beam in one lamp).
• PGJ19-2 — Modern base. Used for H11, H8, H9 and H16 bulbs.
• P20d — Base with 3 plastic pins. Used for the HB3 (9005).
• P22d — Used for the HB4 (9006) (fog light).
• P45t — Old base with 3 pins. Used for the R2 (European Code) on vintage vehicles.

Wedge Base (pinless)

Wedge Base bulbs have a transparent plastic base without pins — the wires come out directly and fold back onto the base.

• W2.1x9.5d — W5W (T10), 10 mm diameter. Side lights, license plate.
• W3x16d — W21W (T20), 20 mm diameter. Turn signals, reverse lights.
• W3x16q — W21/5W (T20), double filament for tail light + brake light.
• W2x4.6d (T5) — Small format, dashboard indicator lights.
• WY2.1x9.5d — WY16W (T15), orange version of T15.

SV Bases (Festoon)

Festoon bulbs have a cylindrical base at both ends. Specific format for interior dome lights, glove compartment lighting, or trunk lighting.

• SV5.5 — Very short format (~31 mm).
• SV7 — Intermediate format.
• SV8.5 — Standard format, the most common (10x31, 10x36, 11x44 mm).

Xenon Bases

Xenon bulbs (HID) use specific bases inherent to this technology:

• D1S — Integrated ballast, mercury.
• D2S — External ballast, mercury.
• D3S — Integrated ballast, mercury-free (Euro 6).
• D4S — External ballast, mercury-free.
• D2R / D4R — Versions with integrated reflector.

How to identify my bulb's base?

Three methods to identify a car bulb base:

1. Consult the vehicle manual — each light is referenced with the bulb type and its base.
2. Remove the burnt-out bulb — the type, wattage (W), and voltage (V) are engraved on the base or the bottom. Example: « H7 PX26d 55W 12V ».
3. Measure the base — diameter and number of pins, pin position (symmetrical vs. offset).

👉 If you can't identify your bulb, consult our complete guide to car lighting or contact us.

Bulb / Base Correspondence Table

Bulb	Base	Function
H1	P14.5s	Low/High Beam
H3	PK22s	Front Fog Light
H4	P43t	Low + High Beam (double filament)
H7	PX26d	Low/High Beam
H11	PGJ19-2	Low Beam (modern)
P21W	BA15s	Turn Signal
P21/5W	BAY15D	Tail Light + Brake Light
PY21W	BAU15s	Orange Turn Signal
R5W	BA15s	Side Light
W5W (T10)	W2.1x9.5d	Side Light, License Plate
T4W	BA9s	Interior Lighting
D1S/D2S/D3S	D1/D2/D3	Xenon Headlight

Difference between automotive and household bases

If you are looking for E14, E27, B22, G9, GU10 bases, you are in the wrong place: these bases are used for indoor bulbs (living room lamps, ceiling lights, household halogens) and are not compatible with vehicles.

Automotive bases are designed to:

• Withstand continuous vibrations and shocks in the vehicle
• Endure high temperatures in headlights
• Ensure watertightness with the socket to prevent moisture
• Comply with R37 standard mandatory for road circulation

FAQ — Car Bulb Bases

What are the different types of car bulb bases?

The main automotive bases are: BA15s and BAY15D (signaling), BA9s and BAY9s (interior, fog), P14.5s (H1), PX26d (H7), P43t (H4), PGJ19-2 (H11), W2.1x9.5d (W5W/T10), SV8.5 (festoon dome lights) and xenon bases D1S/D2S/D3S/D4S.

How do I find out my bulb's base type?

The base type is engraved on the bulb itself (e.g., « H7 PX26d 55W 12V ») or indicated in the vehicle's manual. Failing that, you can remove the burnt-out bulb and compare it with the models available in our catalog.

What is the difference between BA15s and BAY15D?

The BA15s has 2 symmetrical pins and a single filament (P21W, R5W). The BAY15D has offset pins and a double filament (P21/5W) to combine tail light and brake light in one bulb.

Can a bulb be replaced by one with a different base?

No. The base is designed to fit into a specific socket. An incompatible base bulb will not physically fit and risks damaging the socket or causing a short circuit.

Are there car bulb base adapters?

Very rarely. For safety reasons (overheating, incorrect beam positioning), it is strongly advised against using an adapter on a vehicle. Always prefer the original base bulb.

Which bulb for which light?

This depends on your vehicle model. Generally: H4 or H7 for low beam, H1 for high beam, P21W for turn signals, P21/5W for tail light+brake light, R5W or W5W for side lights. Consult your manual or our complete guide.

👉 View all our 12V car bulbs