12 Shape and Diameter of Common Lamps
In the lighting field lamb bulbs are identified with a letter code to identify the overall shape of the bulb, and a numerical designation the represents the widest diameter of that bulb in (1/8) eighths of inches.
For example, a T8 fluorescent lamp would have a tubular (T) shape, and a diameter of 1 inch, or 8 eighths of an inch, while a T12 would have a diameter of 1 and 1/2 inches.
Lamps may also include voltage or power ratings which can be used to make sure that any lamp installed in a lighting fixture will not exceed its rated value of current and reduce the risk of fire or electrical hazards.
EX: A multifaceted reflector, or MR 16 lamp, has a diameter of 2 inches or 16 eighths of an inch.