The false relation

When a tone and its chromatic alteration sound in successive chords, they belong in the same voice: F in the alto followed by F♯ in the alto is a line inflecting; F in the alto followed by F♯ in the soprano is a false relation — two voices contradicting each other about one degree, and the ear flags the contradiction even across the octave. Free styles admit specific exceptions, but the norm stands: chromatic motion is a linear event, and the line that starts it finishes it.

The rule is the router for all chromatic writing: secondary dominants, modal mixture, and chromatic passing motion all work by inflecting a degree, and the inflection reads as intention only when one line carries it. It is the first check when a chromatic progression sounds sour despite correct chords — the alteration usually jumped voices. The deliberate false relations of certain older and newer styles are exactly that: deliberate, exposed, and rationed.

• Splitting an inflection between voices — the natural in one part, the sharp in another — across adjacent chords.
• Producing the clash simultaneously by holding the natural in one voice while another sounds the alteration.
• Hiding the contradiction an octave apart and assuming distance excuses it.
• Re-spelling the alteration enharmonically and treating the contradiction as gone when the ear still hears it.

• Luigi Cherubini, Cours de contrepoint et de fugue (1835), The preliminary rules (the false relation) — The strict-style ban: the chromatic pair belongs to one voice.
• Ebenezer Prout, Harmony: Its Theory and Practice (1889), The chapters on chromatic harmony — The classroom treatment, with the standard licensed exceptions noted.