Altered chords

A key's chords come from the notes of its scale. But composers often replace an expected chord with another to get a fresh harmonic effect, and one way to do it is to change a single note. An altered chord is one of the key's functional chords given another quality because one or more of its notes have been modified (Blatter 2007, part 3, art. 10).

Blatter's example is the simplest possible: the dominant triad turned into a minor chord by lowering its third (Blatter 2007, part 3, art. 10). In C major, G, B, D becomes G, B flat, D.

Med's narrower definition

Med uses the word in a more precise sense. For him an altered chord is built from notes of the altered scale, by raising or lowering diatonic notes, and its mark is a diminished third between two of the chord's notes; the other third is major or minor (Med 1996, ch. XLI). In the altered scale degrees I, III and V are never altered, because they are the ones that define the key.

By Med's criterion, Blatter's G minor is not an altered chord: G, B flat, D is an ordinary minor triad, a minor third and a major third, with no diminished third at all. What makes a chord altered, for Med, is not having an outside note; it is having an interval no diatonic chord has.

A diminished third is the size of a whole tone but is written as a third. Look at the dominant triad of C major and, beside it, the same triad with its fifth lowered:

Hear the passage

From B to D is a minor third. From B to D flat, the same two letters but a semitone less: a diminished third. The chord G, B, D flat has a major third below and a diminished third above, and it is an altered triad in Med's sense.

The four kinds

Since the diminished third can sit below or above, and the other third can be major or minor, there are in theory four altered triads (Med 1996, ch. XLI):

Lower third	Upper third
major	diminished
minor	diminished
diminished	major
diminished	minor

In practice not all of them fit on every degree, because the notes have to come from the altered scale. In C major, Med shows that no altered triads can be built on degrees I and III. And a chord on the fourth degree with an augmented third, rather than a diminished one, does not count (Med 1996, ch. XLI).

The diminished third inverted

A diminished third written in close position sounds odd, cramped, and hardly ever appears that way in music. What appears is its inversion. Inverted, a diminished third becomes an augmented sixth.

Take the chord D, F sharp, A flat, built on the second degree of C major: a major third from D to F sharp, a diminished third from F sharp to A flat. With A flat in the bass, the two notes of the diminished third move apart and form an augmented sixth, A flat below, F sharp above. And each of them lies a semitone from G, one below and one above:

Hear the passage

The two notes open outwards, towards the same note an octave apart. That is why these chords exist: the diminished third, or the augmented sixth it turns into, is an interval made to resolve, with both notes drawn by a semitone. That motion is the basis of some of the best-known chromatic chords in classical music, which have a topic of their own.

Altered seventh chords

The same reasoning holds for seventh chords. Altered seventh chords are formed by altering one or more notes of a diatonic seventh chord, and their mark is still the diminished third: two diatonic thirds, major or minor, and one diminished third. In theory there are twelve combinations, but with the notes of the altered scale fewer are available, and the number changes from degree to degree (Med 1996, ch. LII).

Adding a seventh to the altered D chord gives D, F sharp, A flat, C:

Hear the passage

Major third, diminished third, major third. Because some languages give each altered seventh chord a name of its own, and the names do not match, Med suggests identifying them by their stack of thirds: this is an altered seventh chord "M3 – d3 – M3" (Med 1996, ch. LII). It is the safest way to talk about them, because it does not depend on any naming tradition.

Three senses of "altered"

The word turns up in at least three senses, and it is worth knowing which one a text means.

1. Altered as modified. Any functional chord with a note changed, giving it another quality: Blatter's broad sense (Blatter 2007, part 3, art. 10).
2. Altered as carrying a diminished third. Med's narrow sense, where the sign of an altered chord is the interval, not the note (Med 1996, ch. XLI).
3. Altered dominant. In popular music and jazz, a dominant chord with altered extensions, flat nine, sharp nine, sharp or flat five. It is the sense of symbols like G7(b9), and it has a topic of its own.

The three overlap. The chord D, F sharp, A flat, C is altered in all three senses: it is the dominant of the dominant with its fifth modified, it has a diminished third, and in popular chord symbols it is written D7(b5).

What comes next

The diminished third that becomes an augmented sixth leads straight to the chromatic chords of classical harmony, which use exactly that interval to pull the music towards the dominant. And within a scale, the question of which notes may be altered leads to the chromatic scale.