Introduction of “E series”
E = Edison Screw
Exx = the diameter in millimeters, even in the U.S., where the bulb glass is listed in  ighths of an inch.

(For example, E12 has a diameter o Approx. 12mm.)

There are four common sizes of screw-in sockets used for line-voltage lamps


  Miniature E10

 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)

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  Candelabra E12 North America / E11 in Europe

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.

 

 Intermediate: E17 North America / E14 Europe

Large outdoor Christmas lights use an intermediate base, as do some desk lamps and  many microwave ovens

E17/E14 are also sometimes used, especially in small table lamps and novelty lighting, and occasionally the lights on newer ceiling fans.

 

 Medium or Standard E26 North America / E27 Europe

E26/E27 12V are also produced for recreational vehicles

 

 Mogul: E39 North America, E40 in Europe

The large E39/E40 is used on street lights, and high-wattage lamps and many  non-incandescent high-intensity discharge bulbs.


** Other Applications:

Christmas lights use various base sizes:

E17 for C9 bulbs, E12 for C7 bulbs, possibly E10 for some decades-old series-wired sets in the U.S., and an entirely different wedge base for mini lights.

In countries which use 220–240 volts AC domestic power, E27 and E14 are the most common sizes, although the bayonet mount is also used. In 120-volt North America and 100-volt Japan, the standard size for general-purpose lamps is E26.