Incandescents come in a variety of shapes and sizes. They all fall into a standard system of designation which identifies the bulb shape with a letter, followed by a number. The letter refers to the shape of the bulb; the number identifies the diameter of the bulb (in eighths of an inch). The most common example is an A19 which we know of as a regular bulb. It is designated with the letter “A” because it is an arbitrary design. The diameter of the bulb is 2.375” which is calculated as 2.375 x 8 = 19, therefore the number after the letter “A” becomes 19. Other common types are as follows:

A arbitrary design
B bullet shape
BR bulged reflector
C cone
PAR parabolic reflector
F flame
G globe
MR mini reflector
R reflector
S spherical
T tubular

the letter refers to the shape the number refers to the diameter of the bulb, measured in eighths of an inch.