Whether two voices move into consecutive perfect intervals of the same kind
(fifth-to-fifth, octave-to-octave, unison-to-unison).
Both true parallels (similar motion) and anti-parallels — the same perfect
class reached by contrary motion, e.g. octave to octave with the voices moving
in opposite directions — are flagged, as both are forbidden in strict two-voice
counterpoint. The rule requires that both voices actually move: oblique motion
(either voice stationary) and the no-change case (identical pitches) are excluded.
A fifth expanding to a twelfth counts (same perfect class); a fifth moving to
an octave does not (different perfect kinds — the direct/hidden case owned by
createsHiddenParallelPerfect).
Whether two voices move into consecutive perfect intervals of the same kind (fifth-to-fifth, octave-to-octave, unison-to-unison).
Both true parallels (similar motion) and anti-parallels — the same perfect class reached by contrary motion, e.g. octave to octave with the voices moving in opposite directions — are flagged, as both are forbidden in strict two-voice counterpoint. The rule requires that both voices actually move: oblique motion (either voice stationary) and the no-change case (identical pitches) are excluded.
A fifth expanding to a twelfth counts (same perfect class); a fifth moving to an octave does not (different perfect kinds — the direct/hidden case owned by createsHiddenParallelPerfect).