The 2–3 bass suspension

The suspension turned upside down: here the bass is the voice that holds. Prepared as a consonance, the bass is held while an upper voice moves to stand a second above it on the strong beat; the bass then resolves by falling one step, turning the second into a third and the chord into a sixth chord. Preparation, strong-beat strike, weak-beat fall — the procedure is unchanged; only the direction of listening inverts, because the dissonant voice is now the foundation.

Because the discipline does not care which voice holds, the 2–3 is what a suspension looks like when the bass itself carries the sustained line — a cantus firmus under a moving counterpoint, or a continuo bass syncopated against a part that has already moved on. Its fall to the third makes a sixth chord, a second route to that sonority alongside the 7–6, generated from below rather than above. Fourth species with the counterpoint below the cantus firmus drills exactly this: the bass playing the part an upper voice usually plays.

• Letting the bass move before the strong beat arrives, which releases the preparation early and leaves nothing for the upper voice to strike against.
• Resolving the bass upward or by leap rather than falling one step into the third.
• Doubling the third in an upper voice while the bass is still suspended on the second, the same premature announcement that spoils any suspension, now in the chord’s foundation.
• Doubling the suspended bass tone itself in an upper voice, a heavier error here than above because every other doubling decision is normally measured against the bass.
• Writing the bass as the voice that strikes fresh and an upper voice as the one held over, which produces an ordinary suspension in first inversion, not the bass suspension the 2–3 names.

• Johann Joseph Fux, Gradus ad Parnassum (1725), Liber secundus, fourth species with the counterpoint below the cantus firmus — When the syncopated voice lies beneath, the dissonant tie strikes as a second and falls to the third.
• Ebenezer Prout, Harmony: Its Theory and Practice (1889), The chapter on suspensions — The bass suspension as the inverted form of the upper-voice cases.