12-Bar Blues: What is it? - Page 5

by Jim Burger "OK," you may say, "if my progression is a 'I-IV-V', then why when I play in the key of B is it B-E-F#? Shouldn't it just be B-E-F?" The answer is no!, and here's a layman's explanation of why (my apologies to serious music theory buffs out there).

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:

A: A-D-E
Bb: Bb-Eb-F
B: B-E-F#
C: C-F-G
C#: C#-F#-G#
D: D-G-A
Eb: Eb-G#-Bb
E: E-A-B
F: F-Bb-C
F#: F#-B-C#
G: G-C-D
G#: G#-C#-Eb

By the way, on the guitar another way of looking at it is this -- your I-IV-V is the same as playing your I chord, then playing the same chord barred up five frets (for the IV), then playing it barred up seven frets (for the V). If you know that, then you don't need to understand any of the music theory behind it!