In music, a triad is a set of three notes that can be stacked vertically in thirds. The term "harmonic triad" was coined by Johannes Lippius in his Synopsis musicae novae. When stacked in thirds, notes produce triads. The triad's members, from lowest-pitched tone to highest, are called:
Some twentieth-century theorists, notably Howard Hanson and Carlton Gamer, expand the term to refer to any combination of three different pitches, regardless of the intervals amongst them. The word used by other theorists for this more general concept is "trichord". Others, notably Allen Forte, use the term to refer to combinations apparently stacked of other intervals, as in "quartal triad". In the late Renaissance music era, and especially during the Baroque music era, Western art music shifted from a more "horizontal" contrapuntal approach toward progressions, which are sequences of triads. The progression approach, which was the foundation of the Baroque-era basso continuo accompaniment, required 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. Major and minor triads are the most commonly used triad qualities in Western classical, popular and traditional music. In standard tonal music, only major and minor triads can be used as a tonic in a song or some other piece of music. That is, a song or other vocal or instrumental piece can be in the key of C major or A minor, but a song or some other piece cannot be in the key of B diminished or F augmented. Three of these four kinds of triads are found in the major scale. In popular music and 18th-century classical music, major and minor triads are considered to be consonant and stable, and diminished and augmented triads are considered to be dissonant and unstable.
Construction
Triads are built by superimposing every other note of a diatonic scale. For example, a C major triad uses the notes C–E–G. This 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
minor triads contain a minor third, and perfect fifth, symbolized: R 3 5
diminished triads contain a minor third, and diminished fifth, symbolized: R 3 5
augmented triads contain a major third, and augmented fifth, symbolized: R 3 5
The above definitions spell out the interval of each note above the root. Since triads are constructed of stacked thirds, they can be alternatively defined 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 minor triad A–C–E, 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
augmented triads contain two major thirds stacked, e.g., D–F–A.
Function
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 of the diatonic scale, otherwise symbolized: I, IV, and V. Primary triads, "express function clearly and unambiguously." The other triads of the diatonic key include the supertonic, mediant, submediant, and subtonic, whose roots begin on the second, third, sixth, and seventh degrees of the diatonic scale, otherwise symbolized: ii, iii, vi, and vii. They function as auxiliary or supportive triads to the primary triads.
