The melodic minor arose to rid the harmonic minor of its wide step (the augmented second), awkward for singing. To do so, two degrees are raised at once when moving upward — the sixth and seventh — and the scale becomes almost major in its upper part while keeping the minor third. When moving downward, the classical tradition removes both raises, returning to the natural minor.
This “two-sidedness” makes the melodic minor a flexible expressive means for smooth, singing lines. In jazz the ascending form of the melodic minor is prized on its own — from it descends a whole family of modal scales for improvisation. Together the natural, harmonic, and melodic minor form the threefold system of the minor that every musician uses.
