The letters and numbers on light bulb packages are codes, or shorthand, for the shape and size of the bulb. In other words, they describe, precisely, the shape or type of the bulb, and 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 bulb, with the same shape as many flower bulbs, which 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. or, to be totally confusing, light bulbs that have a "bent tip," which is the exact opposite of a blunt tip. BT, apparently, stands for whatever a company has decided it stand for, and is unreliable as a guide. 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 aren't 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." It is also clearly not a bulb -- it's 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 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" -- 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" in diameter and the newer T-8 tubes are 1" in diameter. A common A-19 light bulb is 2-3/8" in diameter, and an R-30 flood light is 3-3/4" across its widest part.