The fourth against the bass

The perfect fourth leads a double life: between two upper voices it is a consonance and moves freely, but against the bass it is a dissonance and must be treated — prepared and resolved like the suspension it usually is, or passed through in motion. The bass is the reference tone the ear measures everything against, and a fourth above it implies instability rather than a point of rest. One interval, two jurisdictions — which voice is lowest decides the law.

This is the rule that makes the six-four chord special, the 4–3 suspension necessary, and the second-inversion arithmetic of figured bass make sense: every “why is this fourth fine here and forbidden there” question resolves to which voice is on the bottom. It matters practically whenever voices invert — a passage whose fourths were all between upper parts grows dissonances the moment the texture flips and the bass takes the lower tone.

• Treating a fourth above the bass as a free consonance and leaving it unprepared and unresolved.
• Correcting consonant fourths between upper voices as if the bass rule applied to every pair.
• Forgetting the rule when voices invert, so a legal fourth between uppers becomes an untreated dissonance against the new bass.
• Writing parallel fourths against the bass, which are parallel dissonances however smooth they sound between uppers.

• Johann Joseph Fux, Gradus ad Parnassum (1725), Liber secundus, on the status of the fourth — The dialogue settles the fourth’s standing: consonant among the parts, dissonant against the lowest.
• Carl Philipp Emanuel Bach, Versuch über die wahre Art das Clavier zu spielen, Part II (1762), Part II, on the chords containing the fourth — The thoroughbass treatment: the fourth figured, prepared and resolved when the bass holds it up.