The code number of a bulb consists of a letter or letters followed by a number. The letter indicates the shape of the bulb and the number relates to the diameter of the bulb in eighths of an inch. The most commonly used household bulb is the A-19. The bulb is "A" type and the diameter would be 2 3/8" A 120BR40 is a 120 watt reflector 5" in diameter.