Resolving the leading tone

At a cadence the leading tone rises to the tonic — that half-step ascent is the resolution the dominant exists to deliver, and in an outer voice it is obligatory. In an inner voice the tradition grants one license: the leading tone may fall to the fifth of the tonic chord to complete the harmony, because the soprano’s arrival covers the substitution and the ear supplies the rise it expected. The license is for the middle of the texture; an exposed leading tone that fails to rise sounds exactly like the broken promise it is.

The rule decides the last two notes of most soprano lines and the voicing of most final chords: with the leading tone in an outer voice, the close is fixed in advance, and with it inside, the covered license is what lets the final tonic sound complete instead of arriving without a fifth. It is also the first thing to check in a limp cadence — nine times in ten the leading tone went somewhere other than home.

• Letting an outer-voice leading tone fall or leap away, where no covering exists and the ear hears the failure directly.
• Using the covered license and a doubled leading tone together, which no covering repairs.
• Taking the license mid-phrase, where the harmony is not closing and the substituted fifth reads as a wrong note.
• Leaping the leading tone to the third of the tonic chord, resolving the chord but not the tone.

• Jean-Philippe Rameau, Traité de l’harmonie réduite à ses principes naturels (1722), Book II, the perfect cadence — The leading tone’s half-step lean onto the tonic as the engine of the close.
• Ebenezer Prout, Harmony: Its Theory and Practice (1889), The chapters on the dominant harmony — The classroom rule with its inner-voice license stated.
• Hugo Riemann, Vereinfachte Harmonielehre (1893), On the dominant and its leading tone — The function theory’s reading: the leading tone is the dominant’s defining ingredient.