Natural, Harmonic, and Melodic Minor

Major only needs one scale because its pattern of whole and half steps already gives a strong leading tone, a half step below the tonic, pulling the ear home. Natural minor, built from the relative major's key signature with no extra accidentals, has a whole step between its seventh degree and tonic instead. That whole step is too weak to pull a melody back to rest, so composers built two more versions of minor to fix it where needed.

Harmonic minor raises the seventh degree of natural minor by one half step, recreating a true leading tone a half step below tonic. That single change has a side effect: it opens a gap of three half steps, an augmented second, between the raised seventh and the note below it, the sixth degree. That augmented second gives harmonic minor its distinctive, slightly exotic sound, and it is the interval teachers most often ask students to spot and label.

Melodic minor solves the same problem a different way, and only while a melody climbs. Ascending, it raises both the sixth and seventh degrees, smoothing out the augmented second and strengthening the pull to tonic. Descending, that pull is not needed, so the line drops back to the plain natural minor spelling of six and seven. This direction sensitive spelling connects directly to how melodies are analyzed in context: the same key signature can look three different ways depending on which form a passage uses, so recognizing all three is the first step toward reading real minor key music accurately.