In classical theory, there are three types of minor scale. The minor scale we have already discussed, the aeolian mode, is also called the natural minor or pure minor. The two other minor scales were derived from it to provide more interesting harmonic and melodic possibilities. If you construct a ii-V-I progression in a minor key, you will find that the seventh chord built on the root is a minor seventh chord, and the seventh chord built on the second step is a half diminished seventh chord. For example, Am7 and Bm7b5 in the key of A minor. The chord built on the fifth step of this scale is a minor chord, for example Em7 in A minor. The resolution of Em7 to Am7 is not as strong as E7 to Am7. Also, the Am7 does not sound like a tonic; it sounds like it should resolve to a D chord. By raising the seventh degree of the minor scale by a half step (that is, raising the G of A minor to G#), these problems are solved. The chord built on the fifth is now E7, and the seventh chord built on the root is an A minor triad with a major seventh, often notated Am-maj7. This creates a much stronger ii-V-i. The resultant scale, “A, B, C, D, E, F, G#”, is called the harmonic minor, since it is perceived to yield more interesting harmonies than the natural minor.
The seventh degree of a major scale is sometimes called the leading tone, since it is only a half step below the tonic and leads very well into it melodically. The seventh degree of the natural minor scale, on the other hand, is a whole step below the tonic and does not lead nearly as well into it. Although the harmonic minor scale contains a leading tone, if you play that scale, you may note that the interval between the sixth and seventh steps (the F and G# in A harmonic minor) is awkward melodically. This interval is called an augmented second. Although it sounds just like a minor third, there are no scale tones between the two notes. This interval was considered to be dissonant in classical harmony, In order to rectify this situation, the sixth can be raised a half step as well (from F to F#) to yield the melodic minor. In classical theory, this scale is often used ascending only. When descending, since the G# is not used to lead into the tonic A, the natural minor is often used instead. Jazz harmony does not normally distinguish these cases, however. The melodic minor scale “A, B, C, D, E, F#, G#” is used both when ascending and descending.
Both the harmonic and melodic minors outline a m-maj7 i chord, for example Am-maj7 (“A C E G#”) in A minor. Either of the harmonic or melodic minor scales can be used on this chord. The melodic minor is also used on chords marked simply m6, although, as was noted earlier, this symbol can also imply the dorian mode. Several of the modes of the melodic minor scale yield particularly interesting harmonies and are commonly played in jazz. These scales are not commonly described in classical theory, so their names are less standardized than the modes of the major scale.
There is no common term for the second mode of the melodic minor scale. The second mode of A melodic minor is “B, C, D, E, F#, G#, A”. This scale is similar to the phrygian mode except that it has a raised sixth. For this reason it can be called phrygian #6, although that name is not by any means standard. It is most often used as a substitute for the phrygian mode.
The third mode of the melodic minor scale is known as the lydian augmented scale. In A melodic minor, a lydian augmented scale is built on C and consists of “C, D, E, F#, G#, A, B”. This scale contains an augmented major seventh chord “C E G# B”. There is no standard symbol for this chord, but Cmaj7#5 is used occasionally, as is Cmaj7-aug or Cmaj7+. When this chord is called for, the lydian augmented scale is an appropriate choice. The maj7#5 chord is mostly used as a substitute for an ordinary major seventh.
The fourth mode of the melodic minor scale is often called the lydian dominant or the lydian b7. If you construct it, you should see why. In A melodic minor, a lydian dominant scale is built on D and consists of “D, E, F#, G#, A, B, C”. This scale resembles the D major scale “D, E, F#, G, A, B, C#” but with two alterations: the raised fourth characteristic of the lydian mode, and the lowered seventh characteristic of the mixolydian mode. The mixolydian mode was described as a possible scale choice to use over a dominant seventh chord, but the fourth step was an avoid note. The lydian dominant scale does not contain this avoid note. As with the lydian scale and the raised fourth over a major seventh chord, the lydian dominant may sound unusual at first, but it is generally more interesting than the mixolydian when played over a dominant seventh.
This particular sound, the raised fourth over a dominant seventh chord, was widely used in the bebop era, and earned the early bebop musicians a lot of criticism for their use of such non-traditional sounds. This sound was also the genesis of the Thelonious Monk composition “Raise Four”, which prominently features the raised fourth in the melody. The use of this scale is often explicitly indicated by the symbol D7#11. Bebop musicians often called this a flatted fifth, writing the chord symbol as D7b5, although this normally implies the diminished scale, which is discussed later.
The fifth mode of the melodic minor scale has no common name, and is normally used only over the V chord in a minor key ii-V-i progression. This usage will be discussed later.
The sixth mode of the melodic minor is often called locrian #2, since it is actually the locrian mode with a raised second step. For example, the F# locrian mode is based on G major and consists of “F#, G, A, B, C, D, E”, but the F# locrian #2 scale is based on A melodic minor and consists of “F#, G#, A, B, C, D, E”. Since the second step of the locrian mode is an avoid note over a m7b5 chord, the locrian #2 scale is often used instead. This scale is also sometimes called the half diminished scale.
The seventh mode of the melodic minor scale is often called the diminished whole tone scale, because it combines elements of the diminished and whole tone scales discussed later. Another name for this scale is the altered scale. To see why, recall the introductory discussion on chords. Chords are constructed by stacking thirds. Triads consisting of three notes were discussed, as were seventh chords consisting of four notes. In the key of C, G7 is the dominant seventh chord. It contains a root (G), a third (B), a fifth (D), and a seventh (F). If we add another third on top, A, we have a ninth chord G9. If we add another third, C, we have an eleventh chord G11. The C is the fourth of the scale, and is normally an avoid note. This symbol is normally used only when the fourth is explicitly required, as in a suspended chord. If we then add another third, E, we have a thirteenth chord G13. The C is normally omitted from this chord. Another third would bring us back to G.
This chord can be altered by raising or lowering individual notes by a half step. The root, third, and seventh are not normally altered, since they are in large part what define the chord. A change to any of these destroys the dominant feel of the chord. The raised eleventh has already been discussed. The other interesting alterations are to the fifth and the ninth. For a G7 chord, this means the lowered or flat fifth (Db), the raised or sharp fifth (D#), the lowered or flat ninth (Ab), and the raised or sharp ninth (A#).
So now let us return to the so-called altered scale. A G altered scale can be built from Ab melodic minor, and consists of “G, Ab, Bb/A#, Cb/B, Db, Eb/D#, F”. First note that this scale contains G, B, and F, the root, third, and seventh of the G7 chord. The rest of the notes, Ab, Bb, Db, and Eb, are respectively, the flatted ninth, the raised ninth, the flatted fifth, and the raised fifth. In other words, all the possible alterations in a ninth chord are included in this scale. The chord implied by this scale is often notated simply G7alt, although G7#9#5 is used as well, as is G7#9. The b9 and b5 symbols are not normally used in this context, despite being present in the scale, because they imply the diminished scale which is discussed later.
The sound of the altered scale and the chord it implies is much more complex than any other dominant seventh chord/scale so far presented, and it is one of the most important sounds in post bop jazz. You may wish to spend more time on this scale to get used to it. Try going to a piano and playing the root, third, and seventh in your left hand while playing the altered scale, and lines based on it, in your right. You may use this scale even when the chord appears to be an ordinary dominant seventh, but you should do so cautiously in a group setting, because other members of the group may be playing mixolydian or lydian dominant sounds, and your altered scale will sound dissonant against them. This is not necessarily wrong, but you should be conscious of the effect produced.