Voice crossing and overlap

Each voice keeps its registral lane: crossing — an upper voice sounding below a lower one — is avoided in strict writing because the ear assigns lines by register and loses the thread when they trade places. Overlap is the subtler cousin: a voice moving, in one step of the progression, above the note its upper neighbor has just left, or below its lower neighbor’s last note, which momentarily blurs which line went where. Brief, purposeful crossings occur in free styles; overlap between the outer voices is the form the classroom most consistently forbids.

The lanes are what let a listener follow four lines at once, so the rule matters most where the texture is meant to be transparent — chorales, string writing, anything a singer reads. It also has a practical face: parts that cross or overlap are harder to perform from parts, because each musician hears a neighbor where their own line should be. When a crossing is wanted for a melodic reason, it is written knowingly and briefly, not stumbled into while solving a chord.

• Crossing two voices to rescue a doubling or an interval, trading a moment’s correctness for a lost line.
• Overlapping at a phrase join — the tenor rising above the note the alto just left — where the ear reassigns the melody mid-stream.
• Writing crossings that a keyboard realization hides but separate singers or players would expose immediately.
• Letting the bass overlap the tenor at cadences, where the two lowest voices converge and the foundation blurs.

• Luigi Cherubini, Cours de contrepoint et de fugue (1835), The preliminary rules (the crossing of parts) — The Conservatoire statement of lanes: each part keeps its place against its neighbors.
• Johann Georg Albrechtsberger, Gründliche Anweisung zur Composition (1790), The counterpoint chapters, on the conduct of the voices — The strict-composition course’s treatment of register discipline among the parts.