What is the difference between T8, T10, and T12 fluorescent tubes?

Question by travis2500: What is the difference between T8, T10, and T12 fluorescent tubes?
I’m ordering fluorescent tubes, and I see that there are T8, T10, T12, and others. I figured they were different sizes, but then I saw, for example, that you can get medium bi-pin 48″ tubes in both T10 and T12. What does T mean? Thanks
And is F anything important? As in F20, F40.

Best answer:

Answer by Irv S
‘F’ is output Watts.
‘T’ is lamp Dia. in eights of an inch.
(T 10 and T 12 are sometimes interchangable on the same ballast.)

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