There are several different types of electrical bases commonly used on lights. Here's a guide to what's what:
Edison bases: This is the most common type of base, also known as the screw base. It's called the Edison base because it was developed by Thomas Edison and has been used on light bulbs ever since.
Edison bases come in various dimensions, measured in millimeters in diameter.
The common, standard incandescent light bulb uses an E26 base - which means a 26 mm. diameter Edison screw base. It's also known as a "medium" base.
(In Europe and Asia, the standard is 27 mm (ie. E27), but they are interchangeable with E26 bases) GU bases: GU bases are typically two-pin bases. The most common are:
* GU 5.3, which carry 12 volts and are usually found on MR16 lights. * GU10, which carries 120 volts (or 220 volts in Europe), most often seen on MR16-size lights, but using standard line voltage.
Edison screw bases commonly in use are: * standard: E26 or 27 (also known as a "medium" base) * candelabra: E11 or 12
* intermediate (large candelabra): E16 or 17
* mogul: E39 or 40. These are mostly used in high voltage applications.
GU bases: GU 5.3 base. 12 volt two pin base generally found on MR16 lights. Pins are 5.3 mm. apart
GU10 base. 120/220 volt two pin "push & turn" base. Pins are 10mm. apart