For some 500 years in Ancient Greece, the standard music system was called the “Greater Perfect System”. This diagram here shows all of the notes in the diatonic genus of the Hypolydian tonos (or tropos). The full system includes 21 notes per octave, so the genus (whether diatonic, chromatic, or enharmonic) limits each octave to a more manageable 7-note “scale”. The full system also includes 15 different “keys”, so the tonos defines the scale’s lowest bass note (or proslambenomenos), here at around 110Hz. In other words, Aristoxenus would say these notes make up the diatonic genus in the Hypolydian tonos, and Mozart would say they make up a minor scale in the key of A.