chord alteration

In music, an altered chord, an example of alteration (see below), is a chord with one or more diatonic notes replaced by, or altered to, a neighboring pitch in the chromatic scale. For example, the chord progression on the left uses four unaltered chords:

The progression on the right uses an altered IV chord and is an alteration of the previous progression. The A in the altered chord serves as a leading tone to G, which is the root of the next chord.