The Three Minor Harmonic Fields
Natural, harmonic and melodic minor — genesis, crisis and corrections
9.1 The Genesis of the Minor Mode: the Natural Minor Scale and Tonal Relativity
The minor mode does not arise from an isolated structure, but from the concept of tonal relativity. Every major scale has a "sister scale" that shares exactly the same notes and the same key signature, but begins on its sixth degree.
9.2 The Crisis of the Minor Dominant and the Artificial Correction: the Harmonic Minor Scale
To solve the lack of tonal pull and create a force of attraction towards the minor tonic, Baroque composers introduced an artificial chromatic alteration: they raised the seventh degree of the minor scale by a semitone.
9.3 The Melodic Correction of the Augmented Second Interval: the Melodic Minor Scale
Altering the seventh degree solved the harmony problem (the chords), but created a problem for the melody (the singing line). By changing G to G#, the distance between the sixth degree (F) and the seventh degree (G#) became huge: one and a half tones.