Christmas lighting began with small C6 bulbs — where the C means "cone" for the candle flame shape and the 6 indicates the diameter measured in eighths of an inch, ¾ inches (19 mm). These were on a miniature candelabra screw-base, now designated E10 (Edison screw, 10 mm). Replicas of these bulbs are now produced as miniature strings, usually with the entire bulb replaced, but sometimes with a decorative cover with regular bulbs inside. These bulbs tend to be transparent white or colors, and are often ornately designed with crystal-like patterns.
Later bulbs called C7½, have diameters of seven and a half 1/8th inches (15⁄16 in, or 24 mm) using an E12 candelabra base; however, these have a blunt shape (and should therefore be called B7½, or B24). Mixing metric and English units, there are also now G30 globes which are 30 mm (1 3⁄16 in, or G9½) in diameter using these sockets. These are still used for the classic or even retro look, and use about five watts each. Older bulbs drew 7½ watts of power, and were reduced to save power. Early bulbs, as well as some new antique reproductions, are made in various shapes and then painted like Christmas ornaments. Bubble lights and twinkle bulbs also come in this size.
Outdoor-only bulbs are designated C9¼ (1 5⁄32 in, or 29 mm), and have a similar blunt shape as the C7½, but use an E17 "intermediate" base. Some modern versions of these strings are now listed for indoor and outdoor use. These bulbs are rated at about seven watts each, and are available in a globe shape, designated G40 (40 mm, or 1 9⁄16 in). Some of the blunt-shape bulbs now come painted with designs, or swirled in more than one color. It is now very difficult to find twinkle bulbs in this size.
Standard mini lights are T1¾, indicating that they are a tube shape 7⁄32 inches (5.5 mm) in diameter. Larger mini bulbs, which began appearing around 2004, are about twice this size, but are still very uncommon. Both types, along with most of the candle-shaped ones, are pinched-off at the tip rather than the base during manufacturing. Most contemporary miniature light bulbs have an internal shunt that is intended to activate when the bulb's filament burns out. The shunt closes the circuit across the bad filament, restoring continuity which allows the rest of the string to remain lit. However, if one shunt fails to close properly, the whole string will fail to light. Other miniature types include globe-shaped "pearl" and smaller "button" lights, which are often painted in translucent or pearlescent colors. "Rice" lights are tiny, like a grain of rice, and can even have a subminiature base, if they are not already fixed permanently to the wires (on low-voltage sets). Rice lights are typically transparent, although colored variations do exist. They are intended to create tiny points of light, and are suitable for decorating miniature models, small wreaths, and other similar situations in which even "midget" T1¾ lights may be too large.
LED lights, which are encased in solid plastic rather than a hollow glass bulb, may be molded into any shape. Because the LED casts light in only one direction, the most common way LED lights are designed, even "plain" sets having some sort of crystal pattern to create refraction.
Many bargain brands have dome-shaped LEDs which focus the light so that it sharply visible when viewed head-on, but almost invisible from a perpendicular direction. This has both advantages and disadvantages depending on one's decorating needs.
If a small LED bulb size but wider viewing perspective is desired, wide-angle LEDs are available. Rather than being dome-shaped (convex), the envelope is concave (sunken in) to cause wider distribution of light.
All miniature bulbs (including some LED sets) have a wedge base, though their designs are inconsistent with one another, making it somewhat difficult for the average consumer to change bulbs. To replace a bulb, the plastic base of the bulb must usually be changed by straightening the two wires and pulling the glass part out. Most replacement bulbs do not even include the bases anymore, despite getting only ten in a package and being charged nearly half the price of a new string of 100. For this reason, many Americans treat mini Christmas lights as disposable. In addition, colored lights tend to fade with even brief exposure to weather. Many LED sets are permanently wired, with bases that look like conventional pull-out bulbs.
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