Complex sentence: A premise of this definition is that every octave is identically divided by notes selected from the same set of pitch classes and that a "perfect fifth" is the interval between the first and fifth scale degrees of a diatonic major scale, however tuned.
Split into simpler sentences: A premise of this definition is that every octave is identically divided by notes selected from the same set of pitch classes. "Perfect fifth" here means the interval between the first and fifth scale degrees of a diatonic major scale, however tuned.