Difficulty level, roughly compared to ABRSM exam grades. 0 is beginner, 9 is advanced (beyond grade 8).
taken from the foreword:
'This book assumes that the student has a thorough knowledge of the chromatic scale and all major and minor scales and arpeggios. These basic requirements are fully explained in my Clarinet and Saxophone Workbooks. Dominant Seventh and Diminished Chords are revised in this book, owing to their vital importance in jazz.........'