When stacked in thirds, the triad's members, from lowest pitched tone to highest, are called:
- the Root
- the Third - its interval above the root being a minor third (three semitones) or a major third (four semitones)
- the Fifth - its interval above the third being a minor third or a major third, hence its interval above the root being a diminished fifth (six semitones), perfect fifth (seven semitones), or augmented fifth (eight semitones).
Such chords are referred to as triadic.
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.
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, 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 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.
See also 
- Upper structure triad - triads stacked atop other triads
- Pen, Ronald (1992). Introduction to Music, p. 81. McGraw-Hill, ISBN 0-07-038068-6. "A triad is a chord consisting of three notes built on successive intervals of a third. A triad can be constructed upon any note by adding alternating notes drawn from the scale."
- Howard Hanson, Harmonic Materials of Modern Music: Resources of the Tempered Scale (New York: Appleton-Century-Crofts, 1960)
- Carlton Gamer, "Some Combinational Resources of Equal-Tempered Systems", Journal of Music Theory 11, no. 1 (Spring 1967): pp. 37, 46, 50–52.
- Julien Rushton, "Trichord", The New Grove Dictionary of Music and Musicians, second edition, edited by Stanley Sadie and John Tyrrell (London: Macmillan Publishers, 2001).
- Harrison, Daniel (1994). Harmonic Function in Chromatic Music: A Renewed Dualist Theory and an Account of its Precedents, p. 45. ISBN 0-226-31808-7. 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
- fretjam Guitar Theory - Triads on Guitar