Up until this lesson, the only harmony we have covered has been two notes struck together (musical intervals). In reality, the most common harmony in music consists of three notes; for this reason, this type of chord is called a triad. The triad built on the tonic in C major is constructed from scale degrees 1, 3, & 5 (C,E,G), or two thirds stacked on top of the tonic. The bottom note of the triad is called the chordal root, the third above this note is called the chordal third, while the upper note of the triad is called the chordal fifth. Triads can be built on top of every scale degree, so there are actually seven triads in each scale. In order to construct these triads, we must stack two thirds (or a third and a fifth, depending on how you think about this) on top of each scale degree. Each of the resulting triads is named after the scale degree on which it is based (tonic triad, supertonic triad, mediant triad, etc.). Note that the thirds stacked on top of these notes are drawn exclusively from the scale, in this case C major.
The example above shows us the triads on each of the scale degrees in C major; these triads together are known as C major diatonic harmony. The process of finding the diatonic harmony for other keys is just the same: spell the scale, then stack a third and a fifth on top of each scale degree using notes drawn exclusively from the scale. However, there is also a shortcut that can be used to construct diatonic harmony for any major key. The triads on top of each of the seven note names (ACE, BDF, CEG, DFA, EGB, FAC, GBD) remain the same for all keys, although accidentals must be added to each chord when required by the key signature. Therefore, D major would require every F and C to be sharped (DF#A, EGB, F#AC#, GBD, AC#E, BDF#, C#EG), while Gb major would require every B, E, A, D, G & C to be flatted (GbBbDb, AbCbEb, BbDbF, CbEbGb, DbFAb, EbGbBb, FAbCb). Of course, the ordering of the triads would be rotated in order to fix the tonic scale degree and triad in the first position, as in the previous two examples.
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