Screw base light bulbs such as the E26 were introduced by Edison and became widely adopted in the early 20th century. Currently, they are the most common standard in American households.
Edison Screw Base
The most common Edison screw base types are E12 (candelabra), E17 (intermediate) and E26 (medium). The letter "E" designation refers to the Edison screw type, while the number refers to the diameter of the base in millimetres.
Screw base designations are also assigned to bulb sockets. For example, a light fixture with an E26 bulb socket will require an E26 base bulb to work with. E26 is the American standard socket for E26 base bulbs rated for 120V.
E26 and E27
E26 sockets can be used with European standard E27 base bulbs. Both types are interchangeable, and some light fixtures refer to these sockets as E26/E27. However, E27 sockets are rated for 220V; therefore, the voltage compatibility of bulb and light fixture must be checked.
Ref 2 http://donsbulbs.com/cgi-bin/r/b.pl/socket-db-e26-1.html