What is "altered chord"

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. The simplest use of altered chords is the use of "borrowed" chords which are "borrowed" from the tonic minor of a major key, or from the tonic major of a minor key. "Borrowing" of this type is seen in music from the Renaissance music era and the Baroque music era (1600–1750), such as with the use of the Picardy third, in which a piece in a minor key has a final or intermediate cadence in the tonic major chord. "Borrowing" is also common in 20th century popular music and rock music. For example, in music in a major key, such as E major, composers and songwriters may use a D major chord, which is "borrowed" from the key of E minor (where it is the VII chord). Similarly, in music in a minor key, such as A minor, composers and songwriters often "borrow" chords from the tonic major. For example, pieces in B/G minor often use E/C major and F/D major chords (IV and V chords), which are "borrowed" from the key of B/G major.

More advanced types of altered chords were used by Romantic music era composers in the 19th century, such as Chopin and by jazz composers and improvisers in the 20th and 21st century.

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.