B10 refers to bulb shape and means "blunt tip". They can be medium base (E26) or candelabra (E12). E12 means American style candelabra base.

B is "bulged", refers to the shape of the glass portion of the assembly. 10 is the major diameter of the glass part in 1/8ths of an inch, thus 10 is 1 1/4 inches. E12 is the screw thread base size with 12 being the diameter in millimeters. How's that for mixing standards systems?
E26 or E 27 is the size of the screw base of the old standard 60 watt incandescent.

I see! Thank you :)

LOL Nini, I feel your pain. That was me today at Home Depot!

I have a light fixture that claims to come with a T9 bulb. But when I look for replacement T9 bulbs, I am getting circular florescent tubes. The bulbs that came with my fixture look a LOT like T10 bulbs.

So much fun.

T9 incandescents are available. Be sure to put the word "incandescent" in your search terms. Further searching will find the dimensions of the T10. The T10 may be longer. If the T10 will fit the space provided and not exceed the specified wattage, use it.
Often these were used for aquarium lighting.

Here is a link that might be useful: T9

• I'm still confused. Can they be used interchangeably?

E12/E26 is the size of a base, and B10 is the size and shape of the bulb, so you can have a B10 bulb that is B10 and E12, or B10 and E26. So no, they are not "interchangeable", they are two different measurements.

Edited to correct for the mistake JDS mentioned below.

Alex got the lamp shape and base types reversed but got the differences in the bases right.(See bus_driver's comment above)

An E12 is a small "candelabra base" and an E26 is a larger "medium base" like on a standard light bulb. It would not be possible to confuse the real lamps.

well I'm a slow study. are B10 and T10 interchangeable? hey are both allegedly 1.25" but not sure how this all works.

I think a T10 incandescent usually has an E26 (medium) base. A B10 incandescent can have either an E26 (medium) or E12 (candelabra) base. If the base is the same, interchangeability will depend on the space inside the fixture because the T10 is longer.

Lowes doesn't even have any friking sizes on their bulbs

I keep photos and descriptions of every bulb in my Daily Tracker app.

This is a great link to all of the bulb base types including the pin ones that are even more complicated than the screw type ones. :-) http://www.bulbs.com/learning/basechart.aspx

I bought a chandelier that comes with 60W b10.5 candelabra bulbs. Is there any reason why I can’t use a traditional incandescent 60W bulb as long as it has the same base?

Hi everyone I just bought a chandelier from Home Depot that requires a CA-10 bulb and had the same issue as all of you. WHY SO MANY CHOICES!!!!!!! Grrrrrrr. So here is this link hope it helps all of you.. it did to me. So what I found out is the the “CA”is only the shape of the bulb glass part not the metal part. Lol yes I don’t know the technical word for both!!!!

https://www.superbrightleds.com/blog/home-lighting-101-guide-understanding-light-bulb-shapes-sizes-codes/2315/