The Edison screw fitting is a system of screw mounts used for light bulbs, developed by Thomas Edison and licensed starting in 1909 under the Mazda trademark. The bulb has a threaded metal base which screws into a matching socket.
The Edison screw base became popular and eventually displaced competing standards for lamp bases for general lighting purposes in several places, especially in North America and continental Europe. In Britain and countries with a historical connection to Britain, the bayonet cap (BC) is the usual mount. In the early days of electrification, the Edison screw-base socket was the only standardized connector, and even non-lighting appliances sometimes connected to power via a light socket fitting. Today Edison screw lamp bases and fittings are made to dimensions prescribed by international standards and are used for general and specialty lamps.
Early U.S. lamp manufacturers used different and incompatible bases. The Thomson-Houston Electric Company used a threaded stud at the bottom of the socket, and a flat contact ring. The Sawyer-Mann or Westinghouse base used a spring clip acting on grooves in the bulb base, and a contact stud at the bottom of the lamp. By about 1908, the Edison base was most common, with the others falling out of use. 
Dimensions and tolerances for screw bases are standardized in ANSI standard C81.67 and IEC standard 60061-1. The two standards have mostly been harmonized, although several types exist in only one of the two standards with no equivalent in the other.
The designation Exx refers to the diameter in millimeters (i.e., E12 has a diameter of 12 mm.) Do not confuse this with the bulb glass size which in the U.S. is given in eighths of an inch (i.e., MR16, T12). There are four common sizes of screw-in sockets used for line-voltage lamps:
- Candelabra: E12 North America, E11 in Europe
- Intermediate: E17 North America, E14 (Small ES, SES) in Europe
- Medium or standard: E26 (MES) in North America, E27 (ES) in Europe
- Mogul: E39 North America, E40 (Goliath ES) in Europe.
Other screw thread sizes exist for other uses.
The large E39 "Mogul" base is used on street lights, and high-wattage lamps (such as a 100-/200-/300-watt three-way) and many non-incandescent high-intensity discharge bulbs. In areas following the National Electrical Code, general-use lamps over 300 W cannot use an E26 base and instead use the E39 base, 300 W lamps may use either base. Medium Edison screw (MES) bulbs for 12 V are also produced for recreational vehicles. Large outdoor Christmas lights use an intermediate base, as do some desk lamps and many microwave ovens. Previously, emergency exit signs also tended to use the intermediate base, but U.S. and Canadian rules now require long-life and energy-efficient LED lamps, which can be purchased inside a bulb as a retrofit. A medium screw base should not carry more than 25 amperes current; this may limit the practical rating of low voltage lamps.
In countries that use 220–240 volts AC domestic power, standard-size E27 and small E14 are the most common of screw-mount sizes and is prevalent throughout mainland Europe, although the BC bayonet mount or cap is standard light bulb fitting in UK and many British Commonwealth countries. Older installations in some other countries, including France and Greece also use this base. It was first developed by George Lane-Fox in the UK.
E12 is typically used for candelabra fixtures. E17 is also sometimes used, especially in small table lamps and novelty lighting, and occasionally the lights on newer ceiling fans. Christmas lights use various base sizes E17 for C9 bulbs, E12 for C7 bulbs, E10 for decades-old series-wired C6 bulb sets in the U.S., and an entirely different wedge base for T1¾ mini lights. For a short time early on, these mini lights were manufactured using E5 screw bases.
A tiny E5 or E5.5 size is used only for extra-low voltages, such as in interior illumination for model buildings, and model vehicles such as model trains. These are often called "pea bulbs" if they are globe-shaped, but they commonly look like mini Christmas bulbs, or large "grain-of-wheat" bulbs. E10 bulbs are common on battery-powered flashlights, as are bayonet mounts (although those are usually held in with a circular flange located where the base meets the bulb). The E11 base is sometimes used for expensive 50/75/100-watt halogen lights in North America, where it is called the "mini-can", and tighter threads are apparently used to keep them out of E12-base nightlights and other places where they could start a fire.
There are also adapters between screw sizes, and for adapting to or from bayonet caps. A socket extender makes the bulb stick out further, such as to accommodate a compact fluorescent lamp with a self-ballast that don't fit in a recessed lighting fixture.
Most Edison screws have a right-hand threading that tightens when the lamp is turned clockwise. Some lamps and fittings have a left-hand thread to deter theft, since they cannot be used in other light fixtures.
Other uses 
The Edison screw socket was used as an outlet (such as for toasters) when mains electricity was still mainly used for lighting, and before wall outlets became common. Compact fluorescent lamps and LED lamps with Edison bases are made to replace incandescent lamps.
Some adapters for wall outlets use an Edison screw, allowing a light socket to become an ungrounded electrical outlet (such as to install Christmas lights temporarily via a porch light), or to make a pull-chain switch with two outlets, or to split it for two lamps. Another adapter can make a wall outlet into a lamp holder (lamp socket).
Various other accessories have been made, including a smoke detector that recharges over a few hours and lasts for a few days or weeks thereafter, and still allows the attached lamp to operate normally. There have also been electronics that stick onto the end of the screw base and allow the attached lamp to flash, for example, to attract the attention of arriving guests or emergency vehicles; others function as a dimmer or timer, or dim gradually in a child's bedroom in the evening.
|Designation||Base diameter (thread external)||Name||IEC 60061-1 standard sheet|
|E5||5 mm||Lilliput Edison Screw (LES)||7004-25|
|E10||10 mm||Miniature Edison Screw (MES)||7004-22|
|E11||11 mm||Mini-Candelabra Edison Screw (mini-can)||(7004-6-1)|
|E12||12 mm||Candelabra Edison Screw (CES)||7004-28|
|E14||14 mm||Small Edison Screw (SES)||7004-23|
|E17||17 mm||Intermediate Edison Screw (IES)||7004-26|
|E26||26 mm||[Medium] (one-inch) Edison Screw (ES or MES)||7004-21A-2|
|E27||27 mm||[Medium] Edison Screw (ES)||7004-21|
|E29||29 mm||[Admedium] Edison Screw (ES)|
|E39||39 mm||Single-contact (Mogul- in America) Goliath Edison Screw (GES)||7004-24-A1|
|E40||40 mm||(Mogul) Goliath Edison Screw (GES)||7004-24|
Three-way lamps have a d suffix to indicate double contacts, usually E26d or E27d, or rarely E39d. The second contact is used for the lower-wattage filament of the two inside the lamp. This extra contact is a ring located around the main contact. Unlike bayonet sockets, three-way and regular lamps are interchangeable, although the low filament or low setting doesn’t work if mismatched.
The medium Edison screw has seven threads per inch, or about 3.6mm per thread. In the U.S., the Energy Independence and Security Act of 2007 requirement for greater energy efficiency only applies to the medium Edison screw, all other being considered "specialty" lamps.
Diazed fuses DII uses the same E27 thread as lamps, but have a longer body and cannot be screwed into a lamp holder (socket). A lamp base is too short to contact the bottom terminal of a fuse holder. However it's possible (but not useful) to screw a DII fuse holder without a fuse in an E27 lamp holder.
Screw bases suffer from two disadvantages. First, the metal screw itself forms one of the contacts for the circuit. If the lighting system is not correctly wired, or a lamp is plugged into a non-polarized outlet, the metal screw can become energized, presenting an electric shock hazard to anyone attempting to change the lamp. Second, if the lamp unscrews in the socket for any reason, it can lose contact with the center contact and stop working until it is screwed in tighter.
- I.C.S. Reference Library volume 4B, International Textbook Company, Scranton PA 1908, page 43-41
- General Electric Incandescent Lamps manual, publication no. TP 110, 1976 page 12
- ANSI C81.61-2007 American National Standard for electrical lamp bases — Specifications for Bases (Caps) for Electric Lamps, available at www.nema.org, retrieved 2009-01-20