Melodic Minor Scales
I like to think of the melodic minor scale as the chameleon scale as it changes its colours. The ascending scale creates more tension by sharpening the sixth and seventh steps, and the descending scale relaxes that tension by flattening the seventh and sixth steps. The sequence of intervals for the ascending scale of A melodic minor is as follows:
Step 1 – 2 (a – b): whole tone
Step 2 – 3 (b – c): semitone
Step 3 – 4 (c – d): whole tone
Step 4 – 5 (d – e): whole tone
Step 5 – 6 (e – f#) whole tone
Step 6 – 7 (f# – g#) whole tone
Step 7 – 8 (g# – a) semitone
The descending half of the melodic minor scale is identical to that of the natural minor scale:
Step 8 – 7 (a – g) whole tone
Step 7 – 6 (g – f) whole tone
Step 6 – 5 (f – e) semitone
Step 5 – 4 (e – d) whole tone
Step 4 – 3 (d – c) whole tone
Step 3 – 2 (c – b) semitone
Step 2 – 1 (b – a) whole tone
So the ascending scale shares its first five steps with the natural and harmonic minor scales, and its sixth to eighth steps with its major counterpart (note: the major key with the same keynote and NOT the relative major). As already mentioned, the descending melodic minor scale is identical to the descending natural minor scale. We now know that harmonic minor scales form the harmonic basis of minor keys, so it stands to reason (and the name suggests) that melodic minor scales form the melodic basis. The raised sixth step prevents the dissonant augmented second interval found in harmonic minor scales and the raised seventh provides a strong resolution from a leading tone to the tonic. Since descending passages don’t require the tension and definition provided by a leading tone, the descending melodic minor offers a sound truer to the overall minor structure.
The diagram below shows the structure of A melodic minor ascending on the keyboard:
Here’s a video diagram showing the lowest octave of A melodic minor ascending and descending on the cello.
A Bit of History
The development of melodic and harmonic minor scales as we know and use them in Western music happened over a long period. Their predecessors are modes, which date back to ancient civilisations - notably the Ancient Greeks. Mediaeval modes and scales share certain similarities, but follow different rules and form the basis of two different musical languages with distinctly different sounds. It was during the Renaissance period, when polyphonic¹ music really came into its own that the modal system, which had served the simpler homophonic² and monophonic³ musical styles of the Mediaeval period perfectly well, began to prove inadequate, as did the notation system. The rise of polyphony meant that music was becoming considerably more harmonically complex. The need for stronger definition in harmonic resolution drove the development of major and minor keys, and in particular the need for different types of minor scales to cater for a strong leading tone (the raised seventh) and the avoidance of awkward dissonance in melodic vocal lines (the augmented second interval in the harmonic minor scale). Dissonance was a major consideration and was avoided wherever possible in the harmonic structure of renaissance music. For this reason we see elements of all three minor scales in minor keys.
By the early baroque era (from 1600 onwards), a harmonic language based on tonality (harmony based on a key center) rather than modality had emerged. Melodic and harmonic minor scales and major scales were in common use. The range of key signatures increased considerably, and the use of key signatures with sharps was introduced. Equal temperament tuning, a system whereby the octave is divided into twelve equal semitones gained wider acceptance by keyboard makers by the 1630s. Although it did not become the principal tuning system for another two centuries, it enabled the 24 keys found in the circle of fifths - the cornerstone of Western art music from 1600 - 1900.
¹Polyphonic: Musical texture in two or more (usually at least three) relatively independent parts [The Oxford Companion to Music Edited by Alison Latham, 2002]
²Homophonic: Music in which one voice or part is clearly melodic, the others accompanimental and chiefly chordal. The term 'homophony' has also been used to describe part-writing where all parts move in the same rhythm; a more precise term for this is homorhythm. [The Oxford Companion to Music Edited by Alison Latham, 2002]
³Monophonic: A term used to denote music consisting of only one melodic line, with no accompaniment or other voice parts (e.g. plainchant, unaccompanied solo song). [The Oxford Companion to Music Edited by Alison Latham, 2002]
The following table shows major keys, their relative minor keys and the associated key signatures.