Just like the major scale, the minor scale has seven notes. However, it has three variants: the natural minor, the harmonic minor, and the melodic minor. Those variants differ in the manner degrees sixth and seventh are altered.
In the natural minor scale, all notes appear with the same accidentals as in its relative major. Hence the name of natural minor:
Half-steps are found between degrees two and three and between fifth and sixth.
The seventh degree of a minor scale is very often raised. The resulting scale is referred to as harmonic minor scale because the raising of seventh degree is often harmonically motivated. Raising that note forms the dominant chord or dominant seventh chord on the fifth degree of the scale:
Besides raising the seventh degree, the sixth degree may also be altered. The resulting scale is called the melodic minor scale. The main purpose of this accidental is to facilitate the melodic movement from degree sixth to the seventh degree, avoiding the augmented second that is formed in the harmonic minor scale. Therefore, it is referred to as the melodic minor scale:
In traditional music theory the melodic minor scale is presented with the raised VI and VII degrees when the scale is ascending while descending without the accidentals:
In real music we may find the raised degrees ascending and descending.
See S > Scales for related entries