The scales in this section are mostly derived from chord progressions rather than specific chords. For the most part, they can be used as bridges between chords, allowing you to play either the same or very closely related scales over two or more different chords. This is sometimes called harmonic generalization.
The blues scale is often the first scale, after the major scale, taught to beginning improvisers, and is in some cases the only other scale they ever learn. This scale supposedly has its roots in African American music dating back to the days of slavery, but the exact origins of its modern incarnation are unknown. The C blues scale consists of “C, Eb, F, F#, G, Bb”. The second degree of this scale, which is the flatted third of the minor scale, is called a blue note. In vocal music, it is often sung somewhere between an Eb and an E. In instrumental music, various techniques are employed to achieve the same effect, such as stretching the string while playing an Eb on a stringed instrument, lipping down an E on a wind instrument, or striking both the Eb and E simultaneously on a keyboard instrument. The flatted seventh and fifth are also sometimes called blue notes, and are not always sung or played exactly on the notated pitch. Variations on the blues scale that include the natural third, fifth, or seventh can be used as well. Also, note that if the flatted fifth is omitted, the resultant scale is the minor pentatonic scale. The minor pentatonic scale can thus be used as a substitute for the blues scale, and vice versa.
The beauty of the blues scale is that it can be played over an entire blues progression with no real avoid notes. If you try playing lines based on this usage, for instance, a C blues scale over a C7 chord, you get instant positive feedback, since almost everything you can do sounds good. This unfortunately leads many players to overuse the scale, and to run out of interesting ideas quickly. There are only so many phrases (licks) that can be played over a six note scale, and most of them have already been played thousands of times by now. This is not to say you should never use the blues scale; on the contrary, it is vitally important to jazz. But do not become so enamored of the easy gratification it can yield that you practice blues licks over and over rather than expand your harmonic vocabulary.
The language metaphor is a good one. It is hard to say interesting things with a limited vocabulary. Often players like Count Basie are offered as examples of musicians who manage to make a lot out of a little, but there is a difference between saying few words because you are choosing them carefully, and saying few words because you have nothing to say or because your vocabulary is too limited to express your thoughts. This advice transcends the blues scale, of course.
It is not always necessary to vary the harmonic content of your playing if you are sufficiently creative with other aspects. One way to introduce added interest when using the blues scale is to use any special effects at your disposal to vary your sound. This can include honking and screaming for saxophonists, growling for brass players, or using clusters on the piano.
The harmonic minor scale is sometimes played over m-maj7 chords. Its modes have no common names, and they are rarely used by jazz musicians except as bridges over a ii-V-i chord progression. For example, consider the chord progression | Bm7b5 | E7alt | Am-maj7 |. An A harmonic minor scale can be played over all three of these chords, instead of the traditional B locrian, E altered, and A melodic minor scales. Another way of saying this is that the second mode can be played over a m7b5 chord, and that the fifth mode can be used over an altered dominant chord. Even when you are not using the harmonic minor scale over an entire progression, you may wish to use its fifth mode over the V chord in a minor key ii-V-i progression. The advantage of using this scale in this example is that it differs from the B locrian and A melodic minor scales by only one note each. The disadvantage is that the root of the scale is an avoid note in this context.
The melodic minor can be used in this same way; its fifth mode can be used over the V chord in a ii-V-i progression to keep some commonality between the scales used. Note however that the second mode of the A melodic minor is not an ideal choice over the Bm7b5 chord, because this scale has F# instead of F. This is the only difference between the harmonic and melodic minor scales. Your choice of whether to use the fifth mode of the harmonic or melodic minor scales over a dominant seventh chord may partially depend on the key of the tune. If F# is in the key signature, then the melodic minor may sound more diatonic. You may choose that scale if this is the sound you are trying to achieve, or the harmonic minor if you are trying to avoid sounding diatonic. Conversely, if F# is not in the key signature, then the harmonic minor may sound more diatonic. Another issue to consider is which of these scales is closer to the scale you are using on the preceding or following chord. Depending on the sound you are trying to achieve, you may wish to choose the scale that has either more or fewer notes in common with the surrounding scales.
The major bebop scale is a major scale with an added raised fifth or lowered sixth. The C major bebop scale is “C, D, E, F, G, G#, A, B”. This scale can be used over major seventh or major seventh augmented chords. The C major bebop scale can also be used as a bridge between chords in a progression like | Cmaj7 | Bm7b5 E7 | Am |; that is, the same scale can be played over the entire progression. Another way of looking at this is to say that we are playing the C major bebop scale itself over the Cmaj7 chord, playing its eighth mode over the Bm7b5 chord, playing its third mode over the E7 chord, and playing its seventh mode over the Am chord. These modes closely resembly the major, locrian, altered and minor scales respectively. Note that we are using the C major bebop scale over a ii-V-i progression in A minor. In general, we can use the major bebop scale in any given key over a ii-V-i progression in the relative minor to that key.
Other bebop scales include the dominant bebop scale, which is similar to the mixolydian mode but with an additional major seventh. The C dominant bebop scale is thus “C, D, E, F, G, A, Bb, B”. This scale can be used over dominant seventh chords. The major seventh is not really an avoid note if you use it as a passing tone between the C and Bb. It also serves as the raised fourth in the Fmaj7 chord that is likely to follow the C7 chord. There is also the minor bebop scale, which is a dorian scale with an added raised third. The C minor bebop scale is thus “C, D, Eb, E, F, G, A, Bb”. This scale can be used over minor seventh chords, and is often used in minor key blues progressions to give more of a dominant seventh feel to the chords.
The blues and bebop scales are sometimes called synthetic scales, because they do not fit in well with classical theory and appear to have been invented to fit a particular situation. In general, any number of synthetic scales can be constructed using just intervals of minor, major, and augmented seconds. You may wish to try experimenting with developing your own scales and looking for opportunities to use them.