In Theory of Harmony section 4 on the major mode and diatonic chords by Schoenberg, he writes,
A musical sound is a composite, made up of a series of tones sounding together, the overtones; hence, it forms a chord. From a fundamental, C, these overtones are:
c, g, c1, e1, g1, (b-flat)1, c2, d2, e2, f2, g2, etc.
What do the superscripts mean in this context? An octave above the fundamental?