Parallel fifths

Two voices may not move to a perfect fifth from a perfect fifth in the same direction: consecutive fifths. The prohibition is the oldest and firmest in part-writing because two voices locked a fifth apart stop being two voices — the interval is so stable and so bare that the ear fuses them into one thickened line, and the texture loses a voice at the moment both lines claim to move. The rule binds every pair of voices, and compound fifths — twelfths — count the same.

The rule earns its keep wherever two voices move the same way — stepwise harmonizations, sequences, and chord streams are where fifths breed, and checking every pair at every join is the discipline that keeps a texture honest. It also explains repertoire: the moment a style wants voices to fuse rather than converse, from organum to orchestral doubling in fifths, composers break exactly this rule on purpose, which is the strongest proof of what keeping it does.

• Checking only the outer voices, when fifths between any pair — tenor and alto included — carry the same fault.
• Missing compound cases: a fifth moving to a twelfth, or the reverse, is still consecutive fifths.
• Letting a sequence replicate one clean model whose join back to itself makes fifths at every seam.
• Assuming a short ornamental note between two fifths saves them, when the ear still hears the pair.
• Repairing fifths by leaping a voice out of its line, which trades a forbidden interval for a broken melody.

• Johann Joseph Fux, Gradus ad Parnassum (1725), Liber secundus, the four motions and the three rules — Perfect to perfect in similar motion is the case the three rules exclude.
• Luigi Cherubini, Cours de contrepoint et de fugue (1835), The preliminary rules (consecutive fifths) — The categorical nineteenth-century statement of the ban, all voice pairs bound.