·         Here's a very common combination of chords found in jazz progressions (just for a change, the key is F):

/  G-7  /  C7  /  FΔ7  /

·         From / to / represents a bar.  Play this little sequence, using simple shell voicings (Example [3.1]).

·         This motif is often called II-V-I.  That's because the root of G-7 is note II in the scale of F, C is V and F is I.

·         Now look at the shell for the G chord and the shell for the F chord.  Wouldn't it be easy just to move by a small step from G F¹ to F E via, say, F# E (Example [3.2]), instead of jumping up to C B¹b and down again?

·         The answer is - yes.  It happens all the time in jazz.  / G-7 / F#7 / FΔ7 / is a perfectly acceptable alternative to / G-7 / C7 / FΔ7 /.

·         Does this mean that C7 and F#7 are somehow the same chord?  Yes, that's what it means.  And here's one way to explain it:  Example [3.3] is a common variant of the C7 chord.  In this case the G has been flattened - so it's C7 5b.  In bebop they used the flattened fifth chord all the time instead of the straight dominant seventh.

·         Now let's work out what F#7 5b would be (Example [3.4]).  Read off the notes that make up this chord - F# E B¹b C.  Would you believe it - the same notes as in C7 5b, just spread out differently!  So C7 5b and F#7 5b are harmonically identical.

·         In Example [3.5] F#7 5b is used as a chord substitution for the basic C7.  Notice F# is exactly 3 whole tones up from C and three tones down from C¹ just as C divides the interval of six whole tones from F# to F¹# precisely in half.  This three-tone (tritone) relationship is notorious in musical history (it was forbidden in liturgical music, where it was referred to as the "devil's interval").

·         The chord substitution rule is: every dominant 7th chord has a tritone opposite partner (i.e a sharp 4th higher, or a flat 5th lower, same thing).  Work out the shells of all the possible tritone substitutable partners.

·         An important point to notice: in the original II-V-I progression (Example [3.1]) the root notes of the chords (and hence the bass line) move in jumps of fourths and fifths.  In the substituted progression (Example [3.5]) the bass line does not jump - it moves chromatically.

·         In general, jumping (diatonic) basslines and chords are common in pre-bebop jazz (think of stride piano, for example), while chromatic movement is characteristic of modern jazz.  It's a good idea to try and mix the two approaches, for the sake of variety and freshness.