There are a few notable differences between C7 Christmas Light Bulbs and C9 bulbs that you should understand when deciding which one would work best for your intended application. Here are a few to keep in mind.

Main Differences

C7 bulbs are the smaller than C9s. The have an E12 candelabra base, the same size as a night light bulb. They are around 1 1/2 inches long and generally use 5 watt bulbs. These are used for outlining roofs, stringing on pergolas and outlining driveways.

C9 bulbs are larger, over 2 inches long, and fit an E17 base. They are available in 7 or 10 watts and are generally used in the same options as C7'S.

Both C7s and C9s come as incandescent option and will get hot with use. For this reason, it is recommended to keep them away from flammable decorations and especially trees, keep in mind these bulbs should never be used on trees as they are a fire hazard.  They should also be limited to 1000 watts per breaker.

LED varieties

C7 LED light bulbs use 90% less electricity, but are the same size, and can fit into the same size lighting string as incandescents. They have 3 LEDs per base and come in smooth, faceted and dimmable finishes. C9 LED have the same basic attributes as the C7s with 5 LEDs to the base.

If you wish to use either variety in animated light shows, be sure to opt for dimmable bulbs. The LED varieties also have a wider variety of effects and finishes than their incandescent counterparts.

Decorating Ideas for C7 Bulbs

The smaller variety of incandescent-style bulbs is especially good for a high end look for residential applications. They are good for outlining roofs and gazebos all year round, for edging gardens, and wrapping and hanging in trees.

For parties, they create a festive retro look, strung along walkways on stakes or hanging from trees as a kind of canopy. Their vintage look is also a good fit for holiday displays for historic buildings. The opaque colored bulbs even look good unlit.

Decorating Ideas for C9 Bulbs

This larger size of bulb is more often used commercially, for larger scale decorations in malls and stores. The LED C9 bulbs are often used on larger outdoor Christmas trees, and for outlining roofs and doors on commercial property. They are often chosen by churches to light their outdoor Nativity scenes, and by businesses to outline fences and other holiday displays.

A Word About Cords

Whichever bulb you decide works for you, having the right cord is important. While both incandescent lights and LEDs work with the same lighting strings, it's important that you keep in mind the maximum wattage of 1000 watts per breaker, and run no longer than 250 feet of AWG wire no matter how many sockets you have