The number following the E indicates the size in mm of the external thread screw. Thus a E26 has a 26 mm base diameter.

United States – E26 Edison Screw
Canada – E26 Edison Screw
Japan, Korea & Taiwan – E26 Edison Screw
Australia – E27 Edison Screw (or B22 Bayonet Cap)
Europe – E27 Edison Screw
United Kingdom – E27 Edison Screw (or B22 Bayonet Cap)

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.

https://support.lifx.com/hc/en-us/articles/201622624-E26-vs-E27-What-s-the-difference

https://www.earthled.com/blogs/led-lighting-faq-frequently-asked-questions/what-is-the-difference-between-e26-e27-and-a19

http://www.standardpro.com/lamp-bases-in-all-shapes-and-sizes/

http://www.leveck.com/blog/bulb-bases/