Ever heard of the so-called Devils interval? Thats what the tritone is being called. A tritone is an interval of three whole notes (full steps) or six half notes (semitones), thus dividing the octave exactly in half. This interval sounds very dissonant and full of tension and thats the reason why its called the Devils interval. So how does this relate to blues playing? (Blues is The Devil's music ) I have already mentioned that all chords in blues are of the dominant 7th variety. If we take a closer look at a dom 7 chord we will find that the interval between the (major) 3rd and the (dominant) 7 is three whole notes - voila: heres our tritone! In an A 7 chord this would be C# and G. Since G and C# make a tritone, too (a tritone is symetric), these two notes would also be the major 3rd and dom 7th of a D# chord, respectively. (Can you follow me?) This would be called a tritone substitution . We dont want the D# chord in our 12 bar progression (except as a passing chord), but we need the D a half step below and the E a half step above. Before things get too complicated and you get a headache, here are some chord shapes to illustrate how we can apply this: Heres a 3-note A 7 (I) chord: The tritone is on the D and G strings (7th and 3rd), the E on the B string is the 5th here. The D and G strings are fingered with your index and middle, respectively. Heres a 3-note D 9 (IV chord): The tritone is still on the D and G strings, just moved down one fret from A 7, but the 3rd and 7th are reversed, the E on the B string is the 9 of the chord. Heres a 3-note E 7 (V chord): Tritone still on D and G strings, moved up one fret from A 7,the E on the B string is now the I in the chord. Notice that the note on the B string didnt change? We have a pedal tone here. Instead of the E7 we can use the E9 with the same tritone movement but this time the topnote changes to an F# (9 of the E 9 chord): You can use these chord shapes for a stripped down backing adding a lot of suspense and tension to a simple chord progression. Omitting the B string and only fretting the tritones on the D and G strings you can use these double stops very effectively in lead playing, marking your changes between single note lines. .