Why Does a Suspension Belong to Two Harmonies?

The tension of a suspension does not arise simply because a note clashes with the present chord. The note first appears as a normal member of the previous harmony. When the other parts move into a new harmony, it is held over and becomes dissonant; it then usually resolves down by step and is absorbed into the new chord. Preparation, suspension and resolution are all essential. If the dissonance arrives without a history, we hear a collision rather than a note that has been carried across a boundary.

In a common 4–3 suspension, the upper note first sounds consonant with the bass. The bass changes, creating a fourth, and the upper part then moves down to form a third. The expressive point is that the old note is one step late: the new harmony has arrived while a remnant of the old one remains. It preserves memory and creates forward pressure at the same time, so a suspension is not static but a delay with direction.

At the piano, it is easy to give all three stages the same weight. Instead, let the preparation belong naturally to the phrase without announcing tension. Support the suspended note so its friction is audible, but do not replace harmonic pressure with a harsh accent. Let the resolution release that pressure while continuing the melodic line. If the pedal blurs both harmonies together, the suspension loses its identity. Hearing how one note crosses a harmonic boundary matters more than merely memorising the label “4–3”.

https://viva.pressbooks.pub/openmusictheorycopy/chapter/fourth-species-counterpoint/
https://imslp.org/wiki/Well-Tempered_Clavier%2C_Book_1%2C_BWV_846-869_%28Bach%2C_Johann_Sebastian%29