Chord sequence

A chord progression or harmonic progression is a succession of musical chords. Chord progressions are the foundation of harmony in Western musical tradition.

In tonal music, chord progressions have the function of establishing or contradicting a tonality. Arnold Schoenberg distinguishes progressions from successions, which he describes as "aimless". A chord succession, he writes, "may be functionless, neither expressing an unmistakable tonality nor requiring a definite continuation". Chord progressions are usually expressed by Roman numerals.

A chord may be built upon any note of a musical scale, therefore a seven-note scale allows seven basic chords, each degree of the scale becoming the root of its own chord. A chord built upon the note A is an A chord of some type (major/minor/diminished, etc.) The harmonic function of any particular chord depends on the context of the particular chord progression in which it is found. (See Diatonic function)

The diatonic harmonization of any major scale results in three major triads. They are based on the first, fourth, and fifth scale degrees (the tonic, subdominant and dominant – see three-chord song). These three triads include, and therefore can harmonize, every note of that scale.

The same scale also provides three relative minor chords, one related to each of the three major chords. These are based upon the sixth, second and third degrees of the major scale and stand in the same relationship to one another as do the three majors, so that they may be viewed as the first, fourth and fifth degrees of the relative minor key. Separate from these six common chords there is one degree of the scale, the seventh, that results in a diminished chord.

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