The bulb shape of a light bulb refers to the shape and look of it. Different bulb shapes can be used for different lighting fixtures and purposes. Bulb shape is defined by a letter and a number; the letter describes the shape of the bulb and the number indicates the size by the diameter of the bulb in eighths of inches. For example, an A19 bulb would have the shape of a typical light bulb and would be 19/8 of an inch in diameter.
A = Arbitrary Designation: When most people think of a light bulb, they picture this type of light bulb. It is the most widely used light bulb and serves a number of purposes. They are very common in household lamps, but can be used in a variety of lighting applications.
C = Candle: Candle shaped bulbs resemble the shape of a flame on a candle. These bulbs are ideal for chandeliers, but can be used for other lighting purposes, as well. They usually have a smaller base type, such as E12.
CFL Shape: CFL stands for Compact Fluorescent Lamp and is a completely different type of light bulb that uses different technology in order to illuminate. CFL bulbs contain a very peculiar shape where parts of the bulb are spiraled or coiled. Since LEDs use different technology than CFL bulbs, the LED version of this bulb looks different than what a CFL bulb would look like. Instead of spiraling upward, the LED version is more of a cylinder shape.
GU: This bulb shape actually refers to the type of fitting it uses. It is generally used in track lighting fixtures. The letter “G” is often used to describe globe shaped bulbs, which are the extremely round bulbs that are typically found in bathroom lighting fixtures. However, that is not the case with the GU shaped bulb.
MR = Mirror Reflector: These types of bulbs contain a mirror reflector, hence the name, and are fairly wide compared to arbitrary designation bulbs. They can be used for interior accent lighting as well as track lighting.
PAR = Parabolic Aluminized Reflector: These bulbs are called PAR because, similar to the MR shaped bulb, they contain a reflector but in a parabola shape. They can be used in outdoor lighting fixtures for security purposes, but can also be used in recessed downlighting applications indoors.
R = Reflector: Reflector bulbs contain a reflector that drives the light through the top of the light bulb, rather than emitting it all around the bulb as arbitrary designation bulbs do. These bulbs have a similar shape to the A shaped bulbs, but are flatter at the top where they emit their light. They can be used in recessed downlighting or track lighting for indoor lighting applications.
S = Sign: Sign light bulbs have a similar shape as arbitrary designation bulbs, but are slightly more rounded. They are often used to light up signs found on the exteriors of buildings and sometimes come in different colors, other than the various shades of white.
Strip Lighting: Strip lighting is a flexible band that contains tiny little LED lights all along it. It can be used just about anywhere for accent lighting, under cabinet lighting, bar top lighting, display lighting, etc. It can even be placed on vehicles as under body lighting.
T8 = Tube: Tube light bulbs are long, tubular shaped bulbs that are normally used in office lighting applications. Most tube lights are fluorescent lights, but can come in different types of light bulbs.