The melodic augmented second

A single line does not move by an augmented second — or any augmented interval — in the strict style: the interval is hard to sing and harder to hear as melody, its two tones implying different harmonic worlds. The minor mode makes the rule practical: ascending through the sixth and seventh degrees toward the tonic, both are raised — the melodic-minor adjustment — precisely so the line moves by whole and half steps; descending away from the tonic, both revert. Diminished intervals are gentler, permitted when the line turns back inside them; the augmented ones stay out of the melody.

The rule is why the minor mode has three forms in practice: harmonic minor supplies the chord tones, and the melodic adjustment repairs the line whenever a voice must travel through the sixth and seventh degrees. It governs every minor-key cadence approach from below and every bass line that climbs to the leading tone — and knowing it as a repair, not a scale, is what lets a line pick the right accidentals from context.

• Writing the bare harmonic-minor ascent — natural sixth to raised seventh — and producing the augmented second the adjustment exists to remove.
• Raising the sixth and seventh on the way down, where no leading motion justifies them.
• Splitting the adjustment across an ornament, so the augmented second appears between a main note and its neighbor.
• Leaping an augmented fourth or fifth outright, which the strict style bars along with the second.

• Johann Joseph Fux, Gradus ad Parnassum (1725), Liber secundus, the rules of melodic motion — The singable line: steps and plain leaps, the augmented intervals excluded.
• Luigi Cherubini, Cours de contrepoint et de fugue (1835), The preliminary rules (forbidden melodic intervals) — The Conservatoire list of barred melodic intervals, augmented seconds first.