Altered chords

Altered chords are a category of chords that, as the name implies, alter chords by replacing one or more notes. There are, for example, altered dominant seventh chords with a lowered or raised fifth: 7-5 and 7+5.

See diagrams of altered chords:

Theory

By chord alteration means a chord that is modified by a replacement. This is most common for dominant chords and it's mainly the fifth or ninth note that is altered. The alteration is done by changing the note one half step, lowering or raising it.

We can compare a normal dominant seventh chord with altered dominant chords:

C7: C – E – G – Bb
C7-5 (C7b5): C – E – Gb – Bb
C7+5 (C7#5): C – E – G# – Bb

Instead of minus (-), flat (b) is sometimes used and instead of plus (+), sharp (#) is sometime used. Therefore, C7-5 and C7b5 is the same chord and C7+5 and C7#5 is the same chord. Note that C7+5 contains the same notes as Caug7.

Comparing a normal dominant seventh chord with another pair of altered chords:

C7: C – E – G – Bb
C7-9 (C7b9): C – E – G – Bb – Db
C7+9 (C7#9): C – E – G – Bb – D#

The names of these chords are C dominant seventh, sharp ninth and C dominant seventh, flat ninth. C7-9 and C7+9 can also be written C7b9 and C7#9 respectively. A C7-9 can both be seen as a dominant 7th chord with a flattened 9th note or as a 9th chord with a flattened 9th note. Just as C7+9 can both be seen as a 7th chord with a sharpened 9th note or as a 9th chord with a sharpened 9th note.

Comparing with a third group of altered seventh chords:

C7: C – E – G – Bb
C7+11 (C7#11): C – E – G – Bb – F#

The name of this chord is C dominant seventh, sharp eleventh (a correspondent C dominant seventh, eleventh flat ninth does not exist). C7+11 can also be written C7#11. This is an example of an altered chord that includes an added tone.

Less common altered chords

Finally, some less common categories.

Altered ninth and thirteenth chords:

C9#11: C – E – G – Bb – D – F#
C13b9: C – E – G – Bb – Db – F – A
C13#11: C – E – G – Bb – D – F# – A

C9(#11), C13(b9) and C13(#11) can also be written C9+11, C13-9 and C13+11 respectively. Note that C9(#11) also can be seen as a C9(b5).

Some chords include two altered tones. To make their names easier to grasp, they are normally written with the altered notes in brackets.

Altered chords with two altered tones:

C7(#5 #9): C – E – G# – Bb – D#
C13(b9 #11): C – E – G – Bb – Db – F# – A

These chords can be described as a dominant 7th with the fifth sharpened and an added 9th that is also sharpened; a 13th with the ninth flattened and the eleventh sharpened.

When to use alterations?

Especially in jazz, where improvisation is a key element, alterations are used relatively freely as substitutions for dominant 7th chords mainly. Alterations should, however, be avoided if the flatted or sharpened altered note is clashing with the melody.

Chord categories