Secondary chord

Secondary chords in Mozart's Fantasia in C minor, K. 475

Secondary chords are a type of altered or borrowed chord, chords that are not part of the music piece's key. They are the most common type of altered chord in tonal music. Secondary chords are referred to by the function they have and the key or chord in which they function. In Roman numeral analysis, they are written with the notation "function/key". Thus, one of the most common secondary chords, the dominant of the dominant, is written "V/V" and read as "five of five" or "the dominant of the dominant".

The major or minor triad on any diatonic scale degree may have any secondary function applied to it; secondary functions may even be applied to diminished triads in some special circumstances.

Secondary chords were not used until the Baroque period and are found more frequently and freely in the Classical period, even more so in the Romantic period. Composers began to use them less frequently with the breakdown of conventional harmony in modern classical music—but secondary dominants are a cornerstone of popular music and jazz in the 20th century.