12-Bar Blues: What is it? - Page 5
by Jim Burger
The issue with sharps & flats and the I-IV-V is that it's a little misleading to talk about I-IV-V. Here's why: I-IV-V assumes that there are basically 7 notes including your root note (e.g. A-B-C-D-E-F-G if you're in the key of A, so your I-IV-V is A-D-E). But this is an oversimplification -- you've really got 12 notes, not 7 (A-Bb-B-C-C#-D-Eb-E-F-F#-G-G#) We may call it 'I-IV-V', but it's really a "1-6-8" if you look at it over these 12 notes!
So now let's look again at playing a I-IV-V in B. Count 1-6-8 instead, and you'll see that your I-IV-V in the key of B is B-E-F#. Likewise, for F, count 1-6-8 and your I-IV-V progression is F-Bb-C. This yields the following I-IV-V progressions for all 12 possible keys: