Fourth species — the syncope

The syncope: the counterpoint enters on the weak half and ties every note across the bar, so each bar opens with the held tone against the new bass note. If the tied tone remains consonant, the syncope is free and may move on; if it strikes dissonant, it has become a suspension and must fall one step to a consonance. When no workable tie exists, the species is broken for a bar of plain halves and resumed — a licensed repair, not a fault. Fourth species turns rhythm itself into dissonance treatment: the tie displaces the voice against the meter, and the meter is what makes the dissonance intelligible.

The suspension’s training ground, and the species that teaches meter as a musical force: the same two lines, displaced by a half note, suddenly generate chains of tension and release. Its habits transfer directly to figured bass — every 7–6 and 4–3 chain in a partimento is fourth species at the keyboard — and to any style where a voice leans across the barline.

• Resolving a dissonant tie upward or by leap, when it must fall one step.
• Striking the tie on the weak half and resolving on the strong, which reverses the meter the species is built on.
• Forgetting that a consonant tie is free — forcing a stepwise fall where none is owed.
• Refusing to break species where no good tie exists, and accepting a bad tie instead of a bar of clean halves.
• Preparing a tie with a dissonance, when the preparation must be consonant.

• Johann Joseph Fux, Gradus ad Parnassum (1725), Liber secundus, fourth species (the ligature) — Consonant and dissonant ties, the falling resolution, and the licensed break.
• Luigi Cherubini, Cours de contrepoint et de fugue (1835), Fourth species, two-voice counterpoint — The syncope in the Conservatoire course, with the same repair license.