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The Harmonic Minor Scale

In the chapter on the diatonic scale we have briefly touched on the modes.

Modes are scales that share the same notes but start and end on different degrees of the parental scale.

Let's quickly review the modes of the diatonic scale based on the scale degree and the charateristic intervals:
Mode Scale Degree Intervals in the modal scales
prime second third fourth fifth sixth seventh
Ionian  I perfect major major perfect perfect major major
Dorian II perfect major minor perfect perfect major minor
Phrygian  III perfect minor minor perfect perfect minor minor
Lydian  IV perfect major major augmented perfect major major
Mixolydian  V perfect major major perfect perfect major minor
Aeolian  VI perfect major minor perfect perfect minor minor
Locrian  VII perfect minor minor perfect diminished minor minor

Modal playing in the diatonic scale means that the same notes are played, but the tonal center is shifted. To achieve this one can use the following rules of thumb:
1. start on the tonal center of the mode
2. end on the tonal center of the mode
3. repeat the tonal center more often than other notes

Every mode evokes a certain feeling. Experiment with the modes and decide for yourself which ones you like.

While Western classical music definitely makes use of modes sometimes, the main emphasis is on a functional hierarchy that is established by the use of a so called "leading tone".
The leading tone corresponds to the interval of a major seventh relativ to the tonal center or tonic of a scale and leads back to it by moving up a semitone.

If you look at the above mode intervals you will find that only the Ionian and Lydian scales possess a leading tone. Both of these scales are major scales characterized by the invervall of a major third above the tonic. This interval causes a more or less "happy" feeling.

To get the hierarchy of notes and chords that is characteristic of the Ionian or major scale, one has to alter the other scales. However, not every scale's minor seventh has to be raised. For example raising the minor seventh of the Mixolydian scale will just produce another Ionian or major scale. Other scales like Phyrygian and Locrian are rarely used due to the somewhat dissonant interval of a minor second (and a flatted fifth in the case of Locrian), so introducing a leading tone does not make much sense.

This leaves us with only two remaining scales that could benefit from the functional hierarchy of a leading tone: Dorian and Aeolian. They are both minor scales characterized by the invervall of a minor third above the tonic and evoke feelings that are more reminiscent of sadness or reflection.

Let's first take a look at the Aeolian mode or natural minor:
This mode starts on the 6th degree of the C major scale or one minor third below the C of the Ionian or major scale. It is also called the relative minor of C major.

To introduce a leading tone all we have to do is to raise the seventh degree of natural minor. Thus we obtain a major seventh. In the key of A minor this means we have to raise the G to a G# (the accidental # means raising a note by one semitone). This scale is called the harmonic minor scale.
Natural Minor A B C D E F G A
Harmonic Minor A B C D E F G# A

It has got a very characteristic sound with a certain arabic touch that is due to the huge interval between the minor sixth and the major seventh. Since the major seventh function of leading to the tonic is only needed while the melody is ascending, most composers used the natural minor scale when descending, although there is many exceptions to the rule.  

The harmonic minor scale was used a lot in the works of composers from Bach to Mozart but has also seen heavy use in bands like The Doors and recently in the so called "neoclassical" guitar shredding scence. So if you want to spice up your diatonic scale with the sound of a harmonic minor scale you have got to study the scale on the fretboard:

This one note difference compared to the A minor scale seems to be pretty trivial:

But believe me, it is not trivial! This one note change generates almost a new parallel universe...

So how do you learn this scale?

First thing - as always - is to play the scale on only one string using the above fretboard diagram. Thus you will get used to the characteristic intervals on your instrument - the guitar - and will also train your brain to recognize the sound of this particular scale (what is normally called ear-training).

According to the systematic approach we have used with the diatonic scale, it seems then pretty logical to use the same 6-note patterns. This also has the invaluable advantage that we can derive the harmonic minor scale from the natural minor scale wherever we are on the fretboard. And this makes a lot of sense since the harmonic minor scale is generally not used in perfect isolation (unless you are a hardcore "neoclassical" guitarist) but just as a functional extension of the natural minor scale. So never use the harmonic minor scale out of context with the natural minor (or diatonic) scale !

So all we have to do is to transform our 6-note patterns of the diatonic scale to harmonic minor patterns to be able to switch back and forth:
Diatonic Pattern name Diatonic Pattern Harmonic Minor Pattern
Ionian- or C-form   
Dorian- or D-form
Phrygian- or E-form
Lydian or F-form
Mixolydian- or G-form
Aeolian- or A-form
Locrian- or B-form

Due to the huge leap from F to G# these scale patterns are not as user-friendly as the diatonic patterns, but after a while you will get used to it.
Using these patterns you will be able to switch between diatonic and harmonic patterns on the fly - no matter where you are on the fretboard.