Altered chord
A chord with raised or lowered notes.
What it means
An altered chord contains notes that are chromatically raised or lowered against the diatonic stock of the scale. It keeps its function but gains, through the change, a sharper direction: a raised note strains more strongly upward, a lowered one more strongly downward, because both now lie a semitone from their goal.
Musical use
Alteration creates additional leading tones. That is its sense: where the diatonic scale has a whole-tone step, the change produces a semitone step and with it a tension that calls for resolution.
Several altered sonorities have established themselves as fixed formations. The Neapolitan sixth chord is a major triad on the lowered second degree, usually in first inversion, and stands before the dominant; functionally it represents the subdominant. The augmented sixth chord contains, above the lowered sixth degree in the bass, an augmented sixth whose two notes strain apart and resolve into the octave of the dominant; depending on its build it is called Italian, French or German. The dominant with raised or lowered fifth sharpens its pull toward the tonic still further.
Alteration is to be distinguished from the secondary dominant: this is a dominant sonority referring to a degree other than the tonic, and likewise contains notes foreign to the scale, but as a leading tone to that other degree, not as a change of an existing chord note.
In function theory alteration is indicated by additions to the function symbol; the sonority keeps its functional name.
Among the best-known altered chords are the dominant seventh chord with lowered fifth, the augmented sixth chord and the dominant seventh chord with raised fifth. In jazz the abbreviation alt gathers together a dominant seventh chord whose ninth and fifth are raised or lowered; over it the altered scale is played. Common to all forms is that the chromatic notes act as leading tones and reinforce the resolution.
Performance practice
In an altered chord find the changed note and follow where it goes — there the effect lies. Give it weight and lead it on in a directed way. With flexible intonation take raised notes rather high, lowered ones rather low, so that the pulling power becomes audible.