These pieces of music include sections for improvising. A suggested scale is given and in some cases a notated example of the kind of improvisation that you could try.
Flexible Ensemble pieces are suitable for a large variety of combinations. The score is shown in C. The current available parts are shown below the score, but you can also request additional parts for alternative instruments in the relevant transposition (subscribers only
Jazz Flexible Mixed Ensemble - 4 Players Improvisation Pieces