FA-81213 / Music interval and transposition theory / Member archive

Parallel fifths and octaves between two voices: contrary octaves by leap flagged as parallel · case 03

Contrary motion from an octave to a compound octave (C5 over C4 to G5 over G3) is reported as parallel octaves.

Member previewVariant 3 · 3 implementations · 8 checks per implementation

Case contract

Input [upper, lower], equal-length lists of MIDI pitches. For each adjacent pair of positions i, i+1 report [i, "P5"] or [i, "P8"] when the interval (upper - lower) mod 12 is 7 or 0 at both positions (the same class), both voices move (no repeated note in either), and they move in the same direction. Return the list in order; unequal lengths return None.

Why this case matters

Counterpoint checkers and voice-leading validators flag forbidden parallel perfect intervals.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
oracle 0	`[[0, "P8"]]`	`[]`	Failed

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