Every major and minor scale has seven special triads, called diatonic triads, which are formed from that scale's notes.
To discover the diatonic triads, a three step process must be used.
First, construct the scale. We will be using the C major scale for our first example.
Next, stack two generic thirds on top of each note.
Finally, analyze the resulting triads.
The first triad is C – E – G, a major third and a perfect fifth. Therefore, the triad is major.
The second triad is D – F – A, a minor third and a perfect fifth. Therefore, it is minor.
The third triad is E – G – B, a minor third and a perfect fifth. Therefore, it is also minor.
The fourth triad is F – A – C, a major third and a perfect fifth. Therefore, it is major.
The fifth triad is G – B – D, a major third and a perfect fifth. Therefore, it is also major.
The sixth triad is A – C – E, a minor third and a perfect fifth. Therefore, it is minor.
The seventh triad is B – D – F, a minor third and a diminished fifth. Therefore, it is diminished.
The eighth triad is a repetition of the first (C – E – G), making it major.
The first triad of a major scale will always be major, the second and third triads will always be minor, etc.
Next, we will uncover the diatonic triads of the C Natural Minor scale.
First, the scale is constructed. Notice that we are using a key signature rather than placing the accidentals by each note.
Again, stack two generic thirds.
Finally, let's analyze the resulting triads.
The first triad is C – Eb – G, a minor third and a perfect fifth. Therefore, the triad is minor.
The second triad is D – F – Ab, a minor third and a diminished fifth. Therefore, it is diminished.
The third triad is Eb – G – Bb, a major third and a perfect fifth. Therefore, it is major.
The fourth triad is F – Ab – C, a minor third and a perfect fifth. Therefore, it is minor.
The fifth triad is G – Bb – D, a minor third and a perfect fifth. Therefore, it is also minor.
The sixth triad is Ab – C – Eb, a major third and a perfect fifth. Therefore, it is major.
The seventh triad is Bb – D – F, a major third and a perfect fifth. Therefore, it is also major.
The eighth triad is a repetition of the first (C – Eb – G), making it minor.
Next, we will discuss the diatonic triads of harmonic minor.
To convert natural minor to harmonic minor, the seventh tone is raised a half step. Therefore, each Bb (the seventh tone of C minor) is raised to a B.
To convert natural minor to harmonic minor, the seventh tone is raised a semitone. Therefore, each Bb (the seventh tone of C minor) is raised to a B.
Since the third, fifth, and seventh chords have been altered, they need to be reanalyzed.
The third triad is now Eb – G – B, a major third and an augmented fifth. Therefore, it is augmented.
The fifth triad is now G – B – D, a major third and a perfect fifth. Therefore, it is major.
The seventh triad is now B – D – F, a minor third and a diminished fifth. Therefore, it is diminished.
Finally, we will discuss the diatonic triads of melodic minor.
To convert harmonic minor to melodic minor, the sixth tone is raised a half step. Therefore, each Ab (the sixth tone of C minor) is raised to an A.
To convert harmonic minor to melodic minor, the sixth tone is raised a semitone. Therefore, each Ab (the sixth tone of C minor) is raised to an A.
Since the second, fourth, and sixth chords have been altered, they need to be reanalyzed.
The second triad is now D – F – A, a minor third and an perfect fifth. Therefore, it is minor.
The fourth triad is now F – A – C, a major third and a perfect fifth. Therefore, it is major.
The sixth triad is now A – C – Eb, a minor third and a diminished fifth. Therefore, it is diminished.
Use this chart to reference the diatonic triads in each scale.