Light bulbs exist in a range of sizes and forms, and are typically divided into groups according to their bases (the part that screws into a light socket). Every number that begins with the letter E is an Edison bulb.
Thomas Edison created the Edison screw fitting in 1909 as a system of screw mounts for light bulbs. Even non-lighting items (like toasters) were occasionally connected to power via a light socket fitting in the early days of electrification. Today's Edison screw lamp bases and fittings, which are used for both ordinary and speciality lights, are built to the proportions outlined by international standards. The bulb features a metal base with a threaded design that fits into an appropriate socket.
Even in the United States, where the bulb glass is specified in eighths of an inch, the label Exx denotes the diameter in millimetres. (E12, for instance, has a 12 mm diameter.) For line-voltage lighting, screw-in sockets come in 4 standard sizes:
E12 candelabra (diameter of 12 mm)
E17 for intermediate (diameter of 17 mm)
E26 for Medium or Standard (diameter of 26 mm)
Motion: E39 (diameter of 39 mm)
Sizes of Edison Bases
The majority of light bulbs used in the US are E26 in size. It is described as having a "regular" or "medium" basis.
The smaller "candelabra" base is E12. It is used for nightlight bulbs and sometimes for ornamental light bulbs that are placed over bathroom mirrors and in chandeliers.
Between these two sizes is E17, sometimes known as a "intermediate" foundation. It is not as frequently utilised for desk lights and appliance bulbs.
Street lights and high-wattage lighting (such a 100/200/300 Watt three-way) require the huge E39 "mogul" base. Very likely, you won't require the services of these people.
Different usage call for different screw thread diameters.
