I’ve made mention numerous times the importance of the Lydian/ Mixolydian concept regarding dominant tonalities. I’ve mentioned the term ‘alt’, meaning 9’s and 5’s can be raised or lowered at will (again with regard to dominant tonalities). I’ve also mentioned that the diatonic 4th is the ‘avoid note’ within non-minor tonalities, and how to work around that. Of course I’ve stated that rules are meant to be broken. Here’s a great example-- Within any dominant tonality, the flat 6 can be the most interesting note an improviser could choose as a scale tone. Many improvisers (including myself) find the flat 6 MORE interesting when working against the diatonic 4th as opposed to the #4 (lydian/ mixo concept). An interesting comparison is that of the melodic minor vs. harmonic minor. The older jazz greats preferred the smooth sound of melodic minor whereas others went for the sharp angles created by the lowered 6 to major 7 found in the harmonic version.