When you see E26, this is the size of the light bulb base. The E stands for Edison, which is a standard socket for light bulbs and the 26 indicates the diameter size of the base. In the United States, E26 is the most common size of light bulb and referred to as a “medium” or “standard” base. Diameter measurement is in Millimeters not inches! 26 millimeters equals 1 inch.
Based on what you know now, the E39 screw bases are a little larger. E for Edison and 39 for diameter. Not 39 inches!! 39 millimeters – which is still pretty big. These bulbs are also called the “Mogul” size and are primarily used in street lights and lamps that require a higher wattage such the ones found on parking garage canopy lighting or the Metal Halide based wall packs.
E12 is the smaller “candelabra” base. 12mm is equal to 15/32″ but it is commonly used as 1/2″. It’s used for nightlight bulbs, and sometimes for decorative light bulbs used in chandeliers and over bathroom mirrors.
(From Wikipedia) There are four commonly used thread size groups for mains supply 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.