the interval between the sixth and seventh degrees of this scale (in this case f and g) is an augmented second. while some composers, like mozart, have used this interval in melodic composition, other composers found it awkward. they thought that a whole step between these two scale degrees was better for smooth melody writing, so they used the subtonic seventh, or raised the sixth scale degree. these two options are called the ascending melodic minor scale and descending melodic minor scale. the ascending has the same upper tetrachord to the major scale, and the descending is the same as the natural minor: