The letters and numbers on light bulb packages are coded shorthand for the shape and size of each bulb. In other words, they precisely describe the shape or type of bulb as well as its diameter.
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An A designates the teardrop shape of a standard light bulb. This is where the name light bulb comes from. Over time, the phrase light bulb has become used, in general, to describe any source of light that is powered by electricity. In its root meaning, though, this is what it is: an object shaped like a flower bulb that produces light.
Most standard screw-base light bulbs—such as the GE 60 watt incandescent light bulb—are A bulbs. A-19 and A-21 light bulbs, which are most 60 watt and 75 watt light bulbs respectively, are two common examples of A bulbs.
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B can indicate one of three different shapes. When it is used by itself, though, it stands for bullet. A bullet light bulb is a cylinder with a rounded end, like a tubular incandescent bulb, but with a base that is smaller than the glass cylinder.
A BR bulb is a bulged reflector bulb. BR bulbs typically have two funnel-shaped reflector areas, one behind the other, to help shape and focus the light.
A BT code may indicate a bulged tube or chimney light bulb, but it can also stand for a blunt tip light bulb—the ones that are flame-shaped with a rounded end. On the other hand, to be totally confusing, BT light bulbs can also stand for bent tip, which is the exact opposite of a blunt tip. To minimize the confusion, many companies just spell out blunt tip or bent tip on the package rather than relying on the code.
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A C stands for candle. These are the flame-shaped bulbs with the pointed, bent tips. Some companies have started using CA to designate their candle bulbs. That may have started as an attempt to distinguish candle-flame-shaped bulbs from circline lamps.
CIR tells you it's a circular or circline lamp. In other words, it's a fluorescent tube made in a circle.
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G is the code for a globular light bulb. These bulbs are often mounted in the open so they can be seen. One popular location for globular light bulbs is in a multi-socket light fixture above a vanity mirror.Continue to 5 of 8 below.
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An S bulb is made for use in a sign. These are not used in residential lighting.
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A T stands for tube, or tubular. This is the shape of, and the designation for, a standard, straight fluorescent light bulb—though it is also clearly not a bulb in shape but rather a tube. This is also why the electrical trade uses the term lamp instead of light bulb. Lamp has several definitions, of course, but it also has the advantage of not referring to just one shape type within the set of electric-powered light sources.
T-12 and T-8 fluorescent tubes, which are the older and newer versions of straight fluorescent tubes, are two fairly common examples of lamps in this shape. Another is the T-10 tubular incandescent light bulb that is used in the gallery light fixtures that mount to the picture's frame.
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An R lamp is a reflector lamp. If you have any recessed lights or track lights in your home, you probably have some of these lamps installed now.
Variations on the R shape include BR or bulged reflector bulbs, PAR bulbs which have a parabolic shape, and MR or mirrored reflector bulbs.
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The numbers that follow the letter codes are the codes for the size of the light bulb. They show the diameter of the bulb, or lamp, in eights of an inch.
Two familiar examples are T-12 and T-8 fluorescent tubes. The older T-12 tubes are 1 1/2 inches in diameter with the newer T-8 tubes at 1 inch in diameter. A common A-19 light bulb is 2-3/8 inches in diameter, and an R-30 flood light is 3-3/4 inches across its widest part.