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Most of the numbers we use describing chords are odd because harmony is based on thirds. The next number in the 1 - 3 - 5 series is 7. The seventh is always minor unless indicated. A C7 has a B-flat even though the B is natural in the key of C. Cma7 would contain a B-natural. | Sevenths
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| Diminished Sevenths
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A diminished 7th chord uses a diminished triad plus a diminished 7th (half a step smaller than a minor 7th). There are only three such chords, plus the many enharmonic spellings. Sometimes the "correct" spelling of a dim 7th can be awkward. The first two chords here are identical. The other two are simple spellings of the other two dim 7th chords. |
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| Half-Diminished
The notes are E, G, B-flat and D. The name of the chord depends on where it is going. |
The same notes may be analyzed differently depending on the usage. A diminished triad with a minor 7th might be a "half-diminished 7th" as used in the first example. This accounts for the strong dominant - tonic movement in the progression. The middle example is a minor 6th chord. This explains the "Hollywood Cadence" sound of the sub-dominant - tonic progression. The third pair shows the most common usage as a ninth chord missing its root! |
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