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.