Minor scale triads
March 23, 2017 04:07PM
i have a question for the music community. Why is it different building triads of a major scale to a minor scale??? An example of what i mean and makes no sense to me, when using the major scale (we'll use Cmajor) and build triads from each position or mode you end up with - maj, min, min, maj, maj, min, dim. But, when you build them on the Aminor scale there seems to be a sharpened 7th like a harmonic minor scale which gives these triads - min, dim, aug, min, maj, maj, dim. I don't under stand why the use of the harmonic scale when building triads of a minor scale? some help me understand this please.
Re: Minor scale triads
March 24, 2017 01:16AM
That's the minor triads in the harmonic minor system. You could also just have the Aeolian triads which would be min dim maj min min maj maj (min).

Both are valid systems. The reason the harmonic minor sequence exists, and presumably the reason you have been exposed to it) is because the dominant chord of a minor key sounds better if it's a major chord; therefore in the harmonic minor system the V chord is a major instead of the 'default' minor; also the III chord is augmented, so the 5th note is raised; and finally the VII chord starts a semitone higher than you'd expect and consequently is diminished (minor with a flat 5) instead of the expected major. Those 3 changed chords are all targeting that one raised note, the leading note.

It all happens because the harmonic minor concept allows you to have a major V chord in a minor key, that major 3rd acting as a strong leading note to the tonic - ie the I chord's major 7th.

Also have a look at the melodic minor structure which gives a raised 6th as well as a raised 7th, which would give min min aug maj maj dim dim (min). The melodic minor structure exists to have an effective major V chord AND an easier run-up to the tonic note, avoiding the augmented 2nd that harmonic minor has.
Re: Minor scale triads
March 24, 2017 11:46AM
Try not to think of (different) minor scales but of one minor key in which the sixth and seventh notes are variables. - They can be according to key signature or raised by one semitone (half-step).

So in A minor, the available notes are A,B,C,D,E,F,F#,G,G#.

Any of these notes can be found in triads within the key of A minor. That means that D minor (D-F-A) and D major (D-F#-A) are both possibilities for example.

In practice, some of the triads are much more common than others, and some are extremely rare (occurring in music of the Common Practice Period only in very specific circumstances). This is true of the major key too of course; the diminished triad on vii for example is rare in root position.

So, some general rules of thumb to start with:
Augmented triads are seldom used. This means for example that C-E-G#, while theoretically available in A minor, is best avoided.
Diminished triads in root position are also rare, although they may be used in first inversion.

In the minor key, the raised seventh - the "leading note" (G# in A minor) is normally best followed by the tonic.
The raised sixth (F# in A minor) is normally best followed by the raised seventh.

Within a part, augmented intervals are best avoided and the leap of a diminished interval should be followed by notes within that interval. In minor keys, the raised sixth and raised seventh need to be treated carefully with regards to this.

A cadence will be most satisfactory when the dominant chord is major (where applicable). So for example use E major (E-G#-B) in A minor.

For more information, consult a good harmony book such as the one below.

Check out this textbook on Four Part Harmony.
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