Sixth chords ruleM7 = major 6, m7b5 = minor 6

Start from C6 (C, E, G, A). Barry points out that its notes come from two of the three diminished seventh chords: E and G belong to one, C and A to another. The third diminished chord (D, F, A-flat, B) has no notes in C6, so you add it, and C6 plus that diminished chord is the scale: C D E F G A-flat A B. In other words, a major scale with an extra note between the fifth and the sixth.

Introducing the sixth diminished scale1:081:43

The same method works from other chords. Barry notes that the dominant seventh, minor sixth, major sixth and seventh-flat-five chords all come from two of the diminished chords, then asks his secret question: what do you do with the third diminished, none of whose notes are in those chords? Combine it with them. With B7 the third diminished is C-sharp diminished; take the next inversion and you have B major sixth with that diminished, which together make the B major sixth diminished scale.

Jazz theory, part 5: the sixth diminished scale4:265:035:387:03

Every major sixth chord is also a minor seventh chord: C major 6 has the same notes as A minor 7. Barry remembered reading this in his first theory book without seeing why it mattered; later he understood its importance. When you see a minor seventh chord you should know its major sixth, because the major sixth is what makes the sixth diminished scale.

Jazz theory, part 6: every major sixth is a minor seventh0:32Jazz theory, part 4: borrowing diminished notes6:186:53

What the scale gives you, according to Barry: it contains not only the dominant of the major key but also the dominant of the relative minor. It also gives you a scale of chords, which you should know. As he builds it he names the chords in turn: six, diminished, six, diminished.

Introducing the sixth diminished scale2:16Practising the sixth diminished scales0:03

Monk described the first chord of ‘Round Midnight as a minor chord with the sixth in the bass. Barry had learned it as minor seventh flat five, and others call it half-diminished, but for Barry Monk’s description was all it took: play any notes of C minor and put A, the sixth, in the bass, and you have A half-diminished.

Believe: a talk to musicians22:3523:0723:43

Whenever you play a half-diminished chord, know immediately which major key it comes from: it is the relative minor of that major with the sixth in the bass. B-flat half-diminished comes from E major (C-sharp, or D-flat, minor with B-flat below); C half-diminished from G-flat major (E-flat minor with C below); D half-diminished is F minor with the sixth in the bass, so its major is A-flat. Knowing the parent major tells you where the chord came from and how to get there. Barry says standards that go to odd-sounding places, such as I Love You, work this way.

Stella by Starlight10:3011:0212:49Borrowed diminished notes on All the Things You Are16:15

You also have to know how to get to the half-diminished; certain chords lead to certain chords, and you have to play the major to get there. Barry’s route to A half-diminished: start from E-flat major, go to its relative minor, C minor, then put the sixth (A) in the bass. He adds that the chord really belongs to B-flat major, the only place you find it, so you had to change the E-flat major into B-flat major to get to it. He tried it many different ways, and said bass players, for example, do not know these things.

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