Short answer: No, they won’t fit into the same cord because they have different-sized bases.
This is a great question, especially since we’ve had a larger number of first-time Christmas lights customers this season.
We have heard this question in this form several times over the last couple of months along with the question, “Can I have a C9 bulb with a C7 base?” as well.
This blog will address both.
This is a photo of a C7 and C9 glass bulb.
Note that the principles discussed in the post apply equally to standard, commercial C7 and C9 bulbs with threaded bases. We say that because some manufacturers are producing bulbs they are calling C7 and C9 that have push-in bases, and non-standard sized bases that fit into proprietary rather than classic cords.
Note the difference in not just the size of the bulbs but the bases as well.
In the Christmas lights industry. As of this moment, “always”, is the correct word to choose regarding standard, traditional, commercial bulbs.
C7 Bulbs will always have E12 bases. These are also known as candelabra and have the same sized base as a night light bulb.
C9 Bulbs will always have E17 bases. These are also known as intermediate.
The cords are made with either sockets to fit C7 bulbs or C9 bulbs. The different sockets sizes are not mixed on the cords so C7 and C9 bulbs cannot be used on the same strings.
Here are more photos of the bulbs and cords involved.
As an additional note, C7 and C9 bulbs that are any of the following combinations can be mixed as long as they have the same appropriate base size: LED or glass, non-twinkle or twinkle, different shapes like round.
We hope this helps you as you choose between C7 and C9 on your Christmas project design journey.
Here are those photos.