Structure of Chords
The triad is the basic chord of tonal music. Other chords - such as sevenths, ninths, and elevenths - are extension of the triad. Four qualities fo triads are possible: major, minor, augmented, and diminished. The quality of a triad is determined by the kinds of thirds it contains.
Triads are three-note chords build of two superimposed thirds. These two thirds, when stacked on top of each other, create the interval of a fifth between their two outside pitches. You will find that this interval of a fifty is as important to the harmony as it is to the melody. When the triad is written in root position - that is as two superimposed thirds - we identify the three notes of the triad, from the lowest to the highest, as the root, the third and the fifth. In the following example, notice that the third of the triad is an interval of a third above the root, and the fifth of the triad is an interval of a fiffth above the root