Secondary chords are a type of altered or borrowed chord, chords which are not part of the key the piece is in. They are by far the most common sort of altered chord in tonal music. Secondary chords are referred to by the function they have and the key or chord to which they function. Conventionally, they are written with the notation ""function"/"key"". Thus, the most common secondary chord, 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.