Just as triads can be built on the tones of a major scale, they can be constructed on the scale degrees of minor scales as well. There are 3 different minor scale forms – natural minor, harmonic minor and melodic minor. All three forms are used in jazz playing and in jazz writing.
Minor Scale Refresher
Natural minor scale = follows the key signature straight up. The natural minor scale in the key of A minor would subsequently follow the key signature (no sharps or flats) straight up from A to A.
Harmonic minor scale = raising the 7th scale degree of the natural minor. In the case of A harmonic minor, a G now becomes a G# in the scale. This alteration gives the scale a completely different sound. We need this raised 7th to form a major V chord and to help give the key its tonality.
Melodic minor scale = raised 6th and 7th going upwards (in A minor it would include F# and G# going up) and then natural minor scale coming downwards.
This video shows the qualities of triads that can be built on the steps of the harmonic minor scale. Notice how different the qualities of chords are as triads are built on each step of this scale.