Altered chords

Altered chords are a special category of chords that - as the name implies - alter other chords. There are, for example, altered dominant seventh chords with a flattened or a sharp fifth: 7-5 and 7+5.

See diagrams of altered chords:

C7-5 D7-5 E7-5 F7-5 G7-5 A7-5 B7-5

C7+5 D7+5 E7+5 F7+5 G7+5 A7+5 B7+5

Theory

Some theory of this chord category will be presented here.

We can compare a normal dominant seventh chord with altered dominant chords in the note of C:

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

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

Comparing a normal seventh chord with another group of altered chords.

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

The names of these chords are C dominant seventh, sharp ninth and C dominant seventh, flat ninth. You may notice that that is not technically an altering but an addition of notes, but still they are called altered chords. C7-9 and C7+9 could also be written C7b9 and C7#9 respectively.

Comparing with a third group of altered seventh chords.

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

The name of this chord is C dominant seventh, eleventh sharp ninth (a correspondent C dominant seventh, eleventh flat ninth do not exist). C7+11 could also be written C7#11.

Continued reading: Altered chord on Wikipedia