In addition to the natural minor scale, there are two other forms of the minor scale: the harmonic minor and the melodic minor. In this article we will be talking about the harmonic minor scale.
The harmonic scale is a simple derivation of the natural minor scale. All that we need to do to get to the harmonic minor from the natural minor is simply raise the 7th degree by one half step (which is one fret on the guitar, or one key on the piano). For instance, the key of A natural minor includes the notes: A B C D E F and G. To make it harmonic minor we raise the G one half step to a G#, which in A harmonic minor would give us: A B C D E F and G#.
There are a few reasons why we may want to use the Harmonic Minor scale, and one of them is to give us a Middle Eastern feel to our music. The E-F and G#-A sound is very characteristic of this type of music. However, the biggest reason why we use the harmonic minor instead of the natural minor, is that it allows us to sound like we are in the parallel major, particularly when we are dealing with cadences or tension resolution.
Historically, composers like Mozart used the harmonic minor scale to do exactly that. Using the harmonic minor instead of the natural minor allowed them to give stronger resolutions and cadences within their works. And if we look at A minor compared to A major, it makes sense. A major has 3 sharps: F#, C#, and G#. The G# is the most important when dealing with the V (5) chord of A minor and A major (E). By changing G to G#, we not only create a leading tone for A, but we also create a stronger V chord, moving from e minor to E major.
Another way to put it is this: in the A natural minor scale, G and B are a whole step away from the root, A. Sometimes a cadence on “a” (the i chord) will not sound convincing enough without a leading tone, or a half step lead in to the root. In the key of A major, the V chord is E major, which has E G# and B in it. Harmonic minor is essentially swapping the G for a G# to sound more like A major. This is, again, why the harmonic minor was such a staple of the Classical and Romantic era, and why we still use it so frequently today: it allows us to be in a minor key, with all the benefits that entails, but also to get the benefits of the strong V to I relationship that exists in the key of A major. Sometimes, that larger interval from F to G# (an augmented 2nd, equivalent to a minor third) sounds too abrupt, and this is where the melodic minor scale comes in to play.
Our next article will go into the finer points of the melodic minor scale, how it differs from the natural and harmonic minor scales and common uses for the melodic minor scale in modern music.