Hidden fifths and octaves

Similar motion into a perfect fifth or octave — hidden or direct fifths and octaves — is restricted even though the interval is only reached, not repeated: the parallel is implied by the gesture. The rule bears hardest on the outer voices, where the ear tracks the frame: between soprano and bass, similar motion into a perfect interval is acceptable only when the soprano moves by step. Between inner voices, or pairs involving one, the license is broad — the tradition polices the frame, not every pair.

This is the rule that turns the blanket ban on parallels into a usable craft: without its licenses, the standard cadence itself — bass leaping from dominant to tonic under a stepping soprano — would be unwritable. Knowing where the restriction is real (outer voices, both leaping) and where it is a phantom (inner pairs, stepping soprano) saves endless false corrections when checking work.

• Letting both outer voices leap in the same direction into a fifth or octave, the case with no license.
• Flagging every similar approach to a perfect interval as an error, including the stepping-soprano cadence the tradition explicitly allows.
• Hiding the bare arrival at the top of the texture, where an exposed octave between the outer voices rings loudest.
• Forgetting the rule entirely at phrase ends, where similar motion into a final octave is most tempting and most audible.

• Johann Joseph Fux, Gradus ad Parnassum (1725), Liber secundus, on direct motion to perfect consonances — The dialogue’s treatment of approaching perfection in similar motion, and where it is tolerated.
• Luigi Cherubini, Cours de contrepoint et de fugue (1835), The preliminary rules (hidden fifths and octaves) — The codified licenses: outer voices restricted, the stepping upper part excused.