According to this rule, at each change of a (major) chord in the blues, the melodic improviser assumes a change of the scale too with root note the root of the (major) chord! Thus he makes 3 transpositions
Actually this rule may go far beyond the 12 bars blues, to any chord progression!
https://www.youtube.com/watch?v=C8EAAVxvB4A&ab_channel=JamesHargreavesGuitar
