Dyad
- Not to be confused with Dyadic chord.
A dyad (less commonly, diad) is a chord of two pitch classes. In its simplest voicing, a dyad contains two notes, but it may also contain octave doublings (assuming octave equivalence). For example, a power chord is a dyad because it contains only two pitch classes a perfect fifth apart, even though it usually contains at least three notes. Another three-note dyad is the trine of medieval music.
Dyads can be classified by the interval between the notes, and as such the words dyad and interval are sometimes used interchangeably (on the Xen Wiki, especially to distinguish from the mathematical sense of interval (connected subset of the real line)). But in fact dyads and intervals are not the same thing; 2:3:4 is a dyad but not an interval, and 2/1 is an interval but not a dyad.
Just as a triad has first and second inversion, and a tetrad has a first, second and third inversion, a dyad has a first inversion which places the root above the other chord note. A 2-note power chord in first inversion would be voiced as a fourth. Thus a fourth and a fifth, although different intervals, are in fact the same dyad.
Much traditional African music uses dyads almost exclusively, for example the music of the mbira dzavadzimu. One example is Kariga Mombe, beginner version and expert version.