What is a soloist doing when s/he translates a chord progression into an improvised line?
For each chord or sequence of chords it’s a process of building a melodic phase by choosing notes from a particular set that “fits” the harmony. Each set of notes can be thought of as a stepwise succession that leads upward until you get back to the same notes 1, 2, 3 etc octaves higher. Scales in other words. One of the skills a jazz musician develops is to assimilate the connection between a particular progression of chords and the corresponding scale or scales. When this ability is fully internalised and the right scales can be called up more or less instantly as the changes go past, then the improviser has the materials s/he needs to create what is, in effect, a new melody built on the chords of the head.
