·         We've seen how the distinctive qualities of jazz harmony arise from the resolution of different, even opposing, tendencies - chromatic versus diatonic movement (Step 3), tertian and quartal chord construction (Step 11), for example.  Let's look at another source of fruitful tension, the origins of which lie in the cultural history of jazz.

·         The blues is the heart of jazz.  But the logic, harmonic machinery and melodic character of the blues are a challenge to the principles we've been looking at so far.  I've written a bit about this elsewhere on this site, where I suggest a connection between the blues scale and the major bebop scale.  In its bare-bones form, a bebop scale is a conventional major or minor scale with an additional note which has the effect of introducing a chromatic feature to the scale; but most importantly it turns a seven-note into an eight-note scale.

·         Jazz is almost invariably played in even time (that is, multiples of 2 beats to the bar) and an eight-note scale fits the meter and accent pattern of even-time music better than the straight unmodified major or minor scale.  There are whole systems of jazz harmony and improvisation built on the consequences of the bebop modification.  The great pianist Barry Harris is a leading educator in this area.

·         In the spirit of experimentation, you could explore the consequences of adding just one note to, say, the major scale and following the procedure for harmonising the resulting scale by taking each degree (step 1) and building a chord from notes 3, 5 and 7.  For example, the most widely-used major bebop scale is what Barry Harris calls the diminished 6th.  In C, that would work out as C D E F G Ab A B C¹.

·         If you harmonise this, you find an interesting thing.  When 1 = C, the chord made from notes 1 3 5 7 is C E G A, which is C6.  When 1 = D, the chord is D F Ab B, which is Do.  When 1 = E, the chord is E G A C, the first inversion of C6.  For 1 = F, the chord is Fo, which is the first inversion of Do. When 1= G, it's an inversion of C6 again, and so on all the way up - alternating C6 and Do.  Try taking any major or minor scale, adding an extra "bebop note" at different scale degrees and discover the chords that arise when you carry out the harmonisation exercise.  Some quite exotic noises emerge.

·         I've suggested a link between the blues and bebop scales.  The C blues scale is more or less identical with the "mode" of the Bb major diminished sixth scale starting on II.  To generalise - the blues scale I II IIIb IV Vb V VI VIIb shares the same notes as the major dim 6 scale on VIIb.

·         In a way we can see two semi-separate streams running through jazz harmony.  One, coming from the blues with its harmonic character rooted in dominant 7 series and related chords in fourths, has its source in the special qualities of the blue notes (IIIb and VIIb) and generally introduces a gutsy, gritty, bluesy, soulful feel to the music.

·         The other, based on Δ7 and -7 structures and their quartal derivatives, tends to avoid blue notes and is altogether more impressionistic, open and sometimes even a bit fragile in character.

·         We might think of these two conventions as representing, respectively, the Afro-American tradition and Western European custom and practice.

·         A fully rounded, flexible keyboard style will be able to move between these two chordal cultures.  By accessing harmonies of both kinds and putting them together in different combinations, the genteel tendencies of Δ7s and -7s are tempered and invigorated by the astringencies of Afro-American dominants, while the bluntness of the latter can be softened by the delicacies of European voicings.

·         Example [14.1] is a I VI II V I sequence built from tertian "Euro" shells and [14.2] is the equivalent "Afro" progression as quartal 13ths.  We could loop this sequence; and it would quickly become tiresome if we stuck to Euro voicings only, as it would if only Afro chords were employed.  The trick is to mix them up.

·         Examples [14.3], [14.4], [14.5] and [14.6] give, respectively, I, VI, II and V chords in Bb, voiced in various ways.  Can you recognise and name all these structures?  Try putting together chords in different combinations to give I VI II V sequences with different textures.  Get a feel for what works and what doesn't (some combinations, although theoretically acceptable, just sound bad) and look out particularly for nice examples of chromatic or diatonic movement between successive chords.

·         How do I know when and where to take the Afro or Euro (or chromatic or diatonic or quartal or tertian) route when harmonising a jazz song?  The answer is that this is the art of jazz harmony, and like all arts it ultimately comes down to the creativity, judgement and taste of the artist.  The rule is, as ever - trust your ear (and don't get locked in the blues scale jail).