Ninth moving to an octave · Rules
Two voices a ninth apart move to an octave only if the upper voice is the one that moves (the ninth resolving downwards, as in the 9–8 suspension). If the lower voice rises or falls, it is marked.
- Ninth moving to an octave: Two voices a ninth apart move to an octave only if the upper voice is the one that moves (the ninth resolving downwards, as in the 9–8 suspension). If the lower voice rises or falls, it is marked. (Piston, ch. 15, p. 237)