A natural minor key consists of the notes of a minor triad built on the root note, and a diminished minor triad (consisting of two minor thirds) built on the note one major second higher. In addition, there is the note that is a major second (two semitones) above the highest note of the second minor chord.

The natural minor is derived from the early modern aeolian mode. Its semitone steps are between the 2nd and 3rd and 5th and 6th notes.

A harmonic minor key consists of the notes of a minor triad building on the root and a diminished minor triad building on the note one major second higher. In addition, there is the note that is a augmented second (three semitones) above the highest note of the second minor chord.

This makes the chord formed on the dominant of the root a major chord.

A harmonic minor consists of the notes of a minor triad building on the root and a minor triad building on the note one major second higher. In addition, there is the tone that lies a major second above the highest tone of the second minor chord.

This likewise achieves that the chord formed on the dominant of the root becomes a major chord. At the same time, it avoids the augmented second between the 6th and 7th degrees that occurs in harmonic minor, which is considered difficult to sing and unattractive.