In music and music theory, a triad is a three-note chord that can be stacked in thirds. Its members, when actually stacked in thirds, from lowest pitched tone to highest, are called:
Such chords are referred to as triadic.
In the late Renaissance, western art music shifted from more "horizontal" contrapuntal approach toward chord-progressions requiring a more "vertical" approach, thus relying more heavily on the triad as the basic building block of functional harmony.
The root tone of a triad, together with the degree of the scale to which it corresponds, primarily determine a given triad's function. Secondarily, a triad's function is determined by its quality: major, minor, diminished or augmented. Three of these four kinds of triads are found in the Major (or diatonic) scale.
Triads (or any other tertian chords) are built by stacking every other note of a Diatonic scale (e.g. standard major or minor scale). For example, C-E-G spells a triad by skipping over D and F. While the interval from each note to the one above it is a third, the quality of those thirds varies depending on the quality of the triad:
Major triads contain a major third and perfect fifth interval, symbolized: R 3 5 (or 0-4-7 as semitones) play (help·info)
minor triads contain a minor third, and perfect fifth, symbolized: R ♭3 5 (or 0-3-7) play (help·info)
diminished triads contain a minor third, and diminished fifth, symbolized: R ♭3 ♭5 (or 0-3-6) play (help·info)
augmented triads contain a major third, and augmented fifth, symbolized: R 3 ♯5 (or 0-4-8) play (help·info)
The above definitions spell out the interval of each note above the root. Since triads are constructed of stacked thirds, another way to define each triad is as follows:
Major triads contain a major third with a minor third stacked above it. e.g. in the major triad C-E-G, the interval C-E is major third and E-G is a minor third.
minor triads contain a minor third with a major third stacked above it. e.g. in the major triad A-C-E (A minor), A-C is a minor third and C-E is a major third.
diminished triads contain two minor thirds stacked, e.g. B-D-F (B dim)
augmented triads contain two major thirds stacked, e.g. D-F#-A# (D aug).
Each triad found in a diatonic key corresponds to a particular diatonic function. Functional harmony tends to rely heavily on the primary triads: triads built on the tonic, subdominant, and dominant degrees. The roots of these triads begin on the first, fourth, and fifth degrees (respectively) of the diatonic scale, otherwise symbolized: I, IV, and V (respectively). Primary triads, "express function clearly and unambiguously." The other triads of the diatonic key include the supertonic, mediant, sub-mediant, and sub-tonic, whose roots begin on the second, third, sixth, and seventh degrees (respectively) of the diatonic scale, otherwise symbolized: ii, iii, vi, and viio (respectively). They function as auxiliary or supportive triads to the primary triads.
triads on the white piano keys. (file)
^ Pen, Ronald (1992) Introduction to Music, McGraw-Hill, ISBN 0-07-038068-6
^ a b Harrison, Daniel (1994). Harmonic Function in Chromatic Music: A Renewed Dualist Theory and an Account of its Precedents, p.45. ISBN 0226318087. Cited in Deborah Rifkin. "A Theory of Motives for Prokofiev's Music", p.274, Music Theory Spectrum, Vol. 26, No. 2 (Autumn, 2004), pp. 265-289. University of California Press on behalf of the Society for Music Theory