Parallel (consecutive) perfect fifths and octaves

Two voices moving in SIMILAR motion (same direction) from one perfect 5th (or perfect octave/unison) to another perfect 5th (or octave/unison) is forbidden — the voices are perceived as merging into one line. This applies ONLY to similar motion: the identical interval reached by CONTRARY motion (voices moving in opposite directions) or OBLIQUE motion (one voice stationary) is not a parallel-perfect violation and is routine, unremarkable voice leading — most commonly at cadences, where contrary motion into an octave/unison is the PREFERRED cadential approach. This is the one rule every authority agrees on without qualification.

Soprano C4→D4 while Alto F3→G3 (both up a step, both a P5): parallel-5th. Bass C2→D2 while Soprano C4→D4 (both up a step, both an octave): parallel-octave.

Fux/Gould/Read/Ross/Stone/Kostka & Payne/Aldwell & Schachter — universal agreement on the similar-motion case.

This rule can be decided from pitches, durations and voices — no layout required. A validator can check it before anything is drawn.