The Harmonic Minor (lesson)
The Harmonic minor scale is a variation of the minor scale in which the 7th degree is sharpened compared to the natural minor (which make is a major 7th instead of a flattened 7th). This makes the interval between the 6th and 7th notes of the scale an augmented second. The Harmonic minor scale was originally conceived to produce a minor scale that preserved the leading tone of the major scale, which has an important harmonic function but is not present in the natural minor(Aeolian) scale. For more information see my lesson on Minor Scales Revisited.
The resulting scale sounds a little odd and exotic, since the introduction of the major 7th leaves a 3 semitone gap which is a little unusual in western music. In contemporary music, this scale is beloved of neoclassical guitarists and is an important weapon in any one's arsenal. It is a very defined sound and can be overused, but used sparingly it can lend a very exotic feel to an otherwise normal piece.
Number of tones : 7
Intervals : 2-1-2-2-1-3-1
Formula : 1,2,b3,4,5,b6,7
Characteristic Chords : MinorMajor 7, Minor b6