The base of a light bulb (also known as the cap) ensures that the correct bulb type is used in a light fitting. There are a large number of caps now used, both commercially and in domestic situations.
The most common bases are those used with the standard light bulbs, being B15, B22, E14, and E27 where the number denotes the size in 'mm' of the base. These caps are often referred to by their more common names SBC, BC, SES, and ES.
By sales, the next most popular bases seen in a domestic situation are used in the mains and low voltage downlighters. For mains voltage downlighters, most will be seen with a GU10 bulb, in either halogen, compact fluorescent or LED. Some mains downlights will still use the old-style reflector bulbs which have a standard ES cap. You may also have come across a GZ10 cap, which has a bulb designed to reflect heat backward rather than out the front as with a standard GU10. The GZ10 bulb cannot be used in a GU10 fitting. With low voltage downlights, the most popular bulb is the 50mm MR16 which has a GU5.3 base. Also popular are the smaller 35mm MR11 bulbs with their GU4 bases.
Newer and more stylish fittings often make use of capsule bulbs, due to their high output and diminutive size. Similar to the spot bulbs there are mains and low voltage fittings which determine the type of lamp base. For mains voltage, the popular halogen capsule bulbs have a G9 base where the pins are 9mm apart. In low voltage, you will commonly have a G4 of GY6.35 capsule bulbs where again the number denotes the width of the pins.
For under cupboard and strip lighting fluorescent tubes may well be used. These come in 3 main sizes which are denoted by the width of the tube, being T5, T8, and T12. T5 (16mm) tubes have a G5 base, whereas T8 (25mm) and T12 (38mm) tubes have a G13 base. Also used in cupboard lighting are incandescent mains voltage strip bulbs which have an S15 cap (not pictured) or architectural lamps which have an S14d (dual) cap or S14s (single) cap.
Many commercial low-energy fittings use non-integrated compact fluorescent lamps with a push fit connector; being either 2-pin (with internal starter) or 4-pin (requiring external starter). The bases have been designed to allow only the correct-sized bulb to be installed.
Shown below are diagrams of many of the popular bases mentioned. This is by no means an exhaustive list so don't worry if the bulb you have is not represented below. Please get in touch if you require further assistance.