Base Shape Codes:
B or BA = Bayonet
DC = Double Contact
E = Edison Screw
N = Intermediate
G = 2 to 4 Pins
P = Pre-focus
R = Recessed Contact
SC = Single Contact

Base: Provides the electrical contact to the bulb, and in most cases, supports the bulb in the fixture.

Bases are measured in several ways:
Most cylindrical bases are measured by millimeter, 2-Pin bulbs are measured in millimeters center-to-center on the pins.