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 Holiday lighting 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 for incandescent 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 for incandescent bulbs and are generally used in the same ways as a C7 bulb.

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.  On most applications these lights have a socket with fuses.  These sockets usually do not allow for more than 700 watts of power per line.  It is possible to remove the fused socket and get more lights on one strand but this is dangerous unless you know the power capacity of your light stringer.

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 generally have 3 LEDs per base and come in smooth, faceted and dimmable finishes. C9 LED have the same basic attributes as the C7s but usually have 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.  The LED versions of the C7 & C9 strings utilize approximately 1/10th of the wattage of the incandescent versions.  So you can usually utilize about 10 times as many bulbs on one run of LED's vs incandescent options. 

Even though these bulbs do not get hot to the touch we don't recommend utilizing the C7 or C9 bulbs in a tree unless the bulbs are fused into the socket and therefore the bulbs are not removable.  Many people like to hang these type of lights in pine or evergreen trees, but it is possible that one of the pine needles can get into a socket if you are using a non-fused light string. 

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.  Disney World utilizes these bulbs in an LED variety for wreaths, garland and around many of their buildings at the Magic Kingdom. 

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 700 watts per stringer, and run no longer than 250 feet of AWG wire no matter how many sockets you have installed.  Most light stringers are fused so this will not become an issue as the fuse will pop prior to any line melting, but if you are utilizing a non-fused socket of light stringer you need to do the math to know the exact amount of light bulbs you can use per strand.