In Blues soloing part 3, we looked at the mixolydian mode of the major scale in the context of improvising over unaltered dominant chords. Because chords tend to change on the first and third beats of the bar, placing chord tones on these beats really helps to outline the chord progression in your solo. This is fine at slower tempos, but becomes a problem when playing eighth note (quaver) scale passages using the mixolydian. Because there are seven notes in the scale and eight notes per bar, the chord tones become unalligned with the strong beats. This is illustrated in the first example below, which shows an ascending mixolydian eight note passage over two bars.
A solution to this problem is to add an extra note between the root and b7, giving us the bebop dominant scale. Starting on any chord tone and playing eighth notes will keep them alligned with the harmonically strong beats. The second example below shows a bar of the scale starting on each of the chord tones. This scale tends to work better in descending form.