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.