The Edison screw fitting is a system of light bulb connectors, developed by Thomas Edison in 1909 under the Mazda trademark. The designation Exx refers to the diameter in millimeters, e.g. E12 has a diameter of 12 mm.

In the US the standard size is E26, although E12 is used for candelabra fixtures and E10 on some Christmas lights. E17 is also common, especially in some table lamps. In most countries which use 230-240 volts AC domestic power, E27 and E14 are common sizes, although the Bayonet mount is also used.

Larger sizes (E39, E40) are used for higher powered lighting, generally for lamps of over 250 watts.