This unit describes mathematically-oriented properties of the diatonic system for use in introductory music theory courses. It focuses on recent scholarship in the area of diatonic set theory. As students study diatonic music theory, they simultaneously deal with the corresponding mathematical properties that describe aspects of and relationships within a diatonic set in a twelve-note universe. By exploring the mathematical principles behind the unique aspects of the diatonic set, students of music theory can better understand tonal relationships between the notes of the scale and the harmonic significance of these relationships.