Common chord (music)

A common chord, in the theory of harmony, is a chord that is diatonic to more than one key or, in other words, is common to two keys. A "common chord" may also be defined simply as a triadic chord (e.g., C–E–G), as one of the most commonly used chords in a key (I–IV–V–vi–ii), more narrowly as a triad in which the fifth is perfect (i.e., a major or minor triad), in which sense it is alternatively referred to as a "perfect chord" or, more narrowly still (in American practice), as a major triad only.

Common chords are frequently used in modulations, in a type of modulation known as common chord modulation or diatonic pivot chord modulation. It moves from the original key to the destination key (usually a closely related key) by way of a chord both keys share. For example, G major and D major share 4 chords in common: G, Bm, D, Em. This can be easily determined by a chart similar to the one below, which compares chord qualities. The I chord in G major—a G major chord—is also the IV chord in D major, so I in G major and IV in D major are aligned on the chart.