DIATONIC TRIADS

The word diatonic comes from the Greek diatonikós (diá = “through”, tónos = “tone”), and means “to proceed through tones”.

Harmonizing the major scale means associating chords with it, triads in this case. When working with a scale, the safest choice is often to limit ourselves to using only the notes that belong to it. This is what “diatonic” means: using only the notes of the reference scale. Therefore, diatonic triads use only the notes of the scale and fit naturally within it.

On each note of the scale, we can stack two additional notes vertically: a third and a fifth. When adding the third, we must be careful to choose the correct one in order to remain within the notes provided by the scale. For example, if we harmonize the C major scale, we cannot build a major third (+4 semitones) starting from D, because that would produce an F♯, which does not belong to the scale. To harmonize D, the second degree of the major scale, we obtain a minor third (+3 semitones), F, a note that belongs to the scale.

The same logic applies to the fifths: all fifths are perfect (+7 semitones), except on the seventh degree of the scale, where the structure of the scale results in a diminished fifth (+6 semitones).

In general, having only the notes of a scale available to build chords is a useful constraint: notes outside the scale tend to be more difficult to handle.

STACKING IN THIRDS

Instead of counting semitones from the root to find the fifth, we can think in terms of stacking thirds. Starting from C: adding a major third (+4 semitones) gives E; from E, adding a minor third (+3 semitones) gives G, the fifth of C. If we added a major third from E, we would reach G♯/A♭, which lies outside the scale. This reflects a principle of alternation.

Major chords are built by stacking a major third followed by a minor third.Minor chords are built by stacking a minor third followed by a major third.Diminished chords are built by stacking two minor thirds.

I	II	III	IV	V	VI	VII
C	D	E	F	G	A	B
+3a maj	+3a min	+3a min	+3a maj	+3a maj	+3a min	+3a min
E	F	G	A	B	C	D
+3a min	+3a maj	+3a maj	+3a min	+3a min	+3a maj	+3a min
G	A	B	C	D	E	F
=	=	=	=	=	=	=
C+E+G	D+F+A	E+G+B	F+A+C	G+B+D	A+C+E	B+D+F
MAJ	MIN	MIN	MAJ	MAJ	MIN	DIM

A QUICK METHOD

To quickly derive diatonic triads, I can simply visualize the notes of the scale, choose a root note, and, moving upward, skip one note and land on the next to get the third; then skip another note and land on the next to get the fifth: one note on, one note off to obtain the triad.

Starting from the note I’m interested in (which becomes the root), I skip one note of the scale and land on the next — that is the third. Then, from the third, I skip another note and land on the next — that is the fifth.

C MAJOR SCALE DIATONIC TRIADS

CONCLUSION

The most reliable method to harmonize a major scale in triads is to learn the seven types of triads it produces: knowing that degrees I, IV, and V have major triads, degrees II, III, and VI have minor triads, and the VII degree has a diminished triad.

At that point, with some experience, we should be able to recall from memory the major and minor thirds built on each note of the scale, as well as the perfect fifth and the diminished fifth.

Of course, this level of familiarity does not necessarily come from mechanical learning, but can also be the result of years of practice.