The suspension

Dissonance in the strict style is not an event but a procedure with three stations: the tone enters as a consonance (the preparation), is held while the other voice moves so that it becomes dissonant on the strong beat (the suspension proper), and falls one step to a consonance on the following weak beat (the resolution). The strike belongs to the strong beat and the resolution to the weak — reverse them and the syncope loses its meaning. Every suspension — 4–3, 7–6, 9–8 above the bass, 2–3 in the bass — is this one procedure at a different interval.

Fourth species isolates this procedure for practice, because holding a tone into a new harmony is a habit to secure by ear before any single interval is named. It is the generative model behind the four named suspensions and their chains — each the same placement of dissonance on the strong beat, consonance on the weak, at a different interval. Hearing that placement, rather than counting intervals, is what lets strict counterpoint treat dissonance as timing, not as a catalogue of clashes.

• Reversing the two roles — striking the dissonance on the weak beat and resolving it on the strong beat — which turns the syncope into an ordinary figure and erases the accent the device depends on.
• Skipping the preparation and attacking the dissonant tone fresh, so there is no consonant entry for the ear to measure the suspension against.
• Releasing the held tone before the strong beat arrives, which lets the harmony change without the friction the suspension is meant to produce.
• Resolving by leap or by step upward rather than falling one step, which abandons the resolution half of the procedure entirely.
• Treating an interval that is already consonant against the new bass as if it were suspended, so the label is decorative rather than functional.

• Johann Joseph Fux, Gradus ad Parnassum (1725), Liber secundus, fourth species (the ligature) — The species built entirely on this procedure: consonant tie against dissonant tie, and the dissonant tie bound to fall one step.
• Johann Philipp Kirnberger, Die Kunst des reinen Satzes in der Musik (1771), Vol. II, ch. 4 — the placement of dissonance on the accented beat — The strong-beat law: the suspension strikes the accented beat and must resolve on the weak one, never the reverse.
• Ebenezer Prout, Harmony: Its Theory and Practice (1889), The chapter on suspensions — Codifies the three stages and their beat placement for the harmony classroom.