TonalityGUIDE - Tonal music basic theory for Undergraduates
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The triad, along with the scale, is one of the basic units of tonal music. It usually refers to a particular kind of three note chord that consists of two intervals of a third stacked one on top of the other.

The triad constructed from thirds is special because it is the only possible type of three-note chord that is completely consonant. Any other combination of intervals would include a dissonant interval of a second or seventh (or compound versions of these intervals).

The only other consonant three-note chords (i.e. triads containing a third or a fourth) are in fact inversions of triads constructed from thirds.

[many theorists have suggested that the triad is based on the harmonic series]

With two different types of third (major and minor) there are four possible permutations, as shown in the example below. How these function in the tonal system is the main subject of this web site, but it is generally true that major and minor triads are considered more stable in most tonal styles. Augmented and diminished triads usually resolve to a major or minor triad and in some earlier styles only the major triad was considered stable enough to end a whole piece.

lower thirdupper thirdname
1majorminormajor triad
2minormajorminor triad
3majormajoraugmented triad
4minorminordiminished triad