G9 bulbs run at mains voltage (230v). These G9 bulbs have two connectors at the base of the bulb. These connectors are often either two wire loops, some manufacturers use a solid, flat, spade type connector instead.

Most G9 bulb manufacturers embed the electrical connectors directly into the glass base of the bulb, but some add a ceramic base too.

G9 bulbs are most often used in household decorative lighting, where they are most often found inside a glass light shade. Their small size and high light output means that they can be used in main room lights.

G4 halogen capsule bulbs are low voltage voltage bulbs. e.g. 6v, 12v, 24v, 28v. G4 bulbs run at a low wattage, with the maximum being 20w. The pins of a G4 bulb are 4mm apart.

The 12 volt G4 bulbs are very common in household lighting, often used in decorative light fittings which have multiple bulbs. They are also used in some small utility lights such as under cabinet lighting.

The other voltages are less common and are used in a variety of applications from torches, to bicycle lamps to commercial vehicles and boats.

GY6.35 bulbs are low voltage halogen capsule bulbs, but generally run at a much higher wattage than G4 bulbs.

Like a G4 bulb, GY6.35 bulbs also have two (often sharp) pins at the base but as their name suggests they are further apart at 6.35mm.

The higher wattage versions of these bulbs are often used in specialist applications such as projectors and stage lighting.