In music and music theory, a triad is a three-note chord that can be stacked in thirds.^[1] Its members, when actually stacked in thirds, from lowest pitched tone to highest, are called:

• the Root
• the Third (whose interval is a major third or minor third above the root)
• and the Fifth (whose interval is a major or minor third above the third, and a diminished, perfect, or augmented fifth above the root).

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 the Major (or diatonic) scale.

Triads (or any other tertian chords) are built by stacking every other note. 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)

Each triad found in a diatonic key corresponds to a particular diatonic function. Functional harmony tends to depend heavily on the tonic, subdominant, and dominant degree triads. The roots of these triads begin on the first, fourth, and fifth degrees (respectively) of the diatonic scale, otherwise symbolized: I, IV, and V (again, respectively). 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 vii^o (again, respectively). They function as auxiliary or supportive triads to the primary triads.

See also

• Tertian harmony
• For information on 3-note chords stacked in intervals other than thirds (e.g., fourths and fifths), see Trichord
• Upper structure triad

References

1. ^ Pen, Ronald (1992) Introduction to Music, McGraw-Hill, ISBN 0-07-038068-6