A halogen capsule is defined by its base, not its glass. The base determines which socket it fits, what voltage it can safely run, and in many cases how long it survives. Yet "halogen bulb" on a purchase order without a base type is the single most common reason replacement orders end up as warehouse shelf-fillers.
G4 means two pins spaced 4mm apart. It is a low-voltage capsule — 6V, 12V or 24V — which means the fixture must contain a transformer. You will find G4 in under-cabinet lighting, chandeliers and microscope illuminators. The capsule body is roughly 33 × 10mm, and wattages run 5W to 35W. The giveaway when buying: if the lamp came with a small transformer or the socket has only two thin holes, it is G4 territory.
G8 uses two pins spaced 8mm apart and runs at mains voltage, usually 120V. The wider pin spacing exists precisely so a G8 cannot be forced into a G4 socket and run at the wrong voltage. G8 capsules are about 42 × 12.8mm, commonly 20W, 35W and 50W, found in accent and display lighting where a slightly larger capsule is acceptable.
G9 is a loop base — two pins joined in a loop that wraps around the glass — rated for mains voltage at 110V or 220V. The loop design handles thermal expansion, which is why G9 oven capsules dominate oven and appliance lighting. Body size is 40 × 12.8mm, wattages 18W to 60W. If your lamp sits behind an oven door or in a range hood, it is almost certainly G9.
GU10 is the twist-and-lock base: two pins plus a wider flange that locks into the socket with a quarter turn. It is strictly mains-voltage, used in spotlights, track lights and recessed downlights. GU10 capsules run 20W to 100W. You can identify it instantly: the base has visible ribs and locks with a twist, unlike the push-in G4/G8/G9.
The identification sequence that never fails: look at the socket first. Two thin push-in holes → measure pin spacing → 4mm is G4, 8mm is G8. A loop base → G9. A twist-and-lock flange → GU10. Never guess from the fixture brand or the old lamp’s appearance alone — glass shapes vary even within one base type.
If you are sourcing in volume, buy by base type and voltage written explicitly on the PO, and ask the factory to confirm compatibility with your fixture drawing. Shunqi Lighting manufactures the full G4, G8, G9 and GU10 range across 110V and 220V, and our engineers will confirm the correct base for your fixture before you commit.