 In the designation "Exx", "E" stands for "Edison" and "xx" indicates the diameter in millimeters as measured across the peaks of the thread on the base (male), e.g., E12 has a diameter of 12 mm.This is distinct from the glass envelope (bulb) diameter, which in the U.S. is given in eighths of an inch, e.g., A19, MR16, T12.There are four commonly used thread size groups for mains supply lamps