Choro harmony, and tonal grammar taken to its limit
Chained dominants, passing diminished chords, a bass always in motion and modulation between sections; the Brazilian repertoire where nineteenth-century functional harmony survived most intact, and why memorising the form is worth more than memorising the chords.
Choro is nineteenth-century functional harmony played by people who never stopped playing it. Secondary dominants in a row, diminished chords linking neighbouring degrees, the bass walking all the time, modulation to a neighbouring key every section: everything the classical harmony manuals describe is there, at a fast tempo, in a fixed form. This page reads that repertoire.
Chained dominants
A choro phrase rarely goes from one degree to another without preparing. Every arrival chord gets its dominant, and the dominants chain by fifths all the way to the tonic.
The second is the pure chain: A, D, G, C, each chord the dominant of the next. It is the skeleton of countless first sections, and the bass falls by fifths the whole way.
The passing diminished
Between two degrees a whole tone apart, choro puts the diminished chord on the note between and links the two by semitones. It is the trademark of the accompaniment, and it is a secondary dominant without a root.
In the second, the diminished chord on the same root as the chord before it: D minor becomes D diminished and resolves to C major in first inversion. It is the diminished "from above", which falls instead of rising, and the bass D, D, E walks by step.
The bass that never stops
The seven-string guitar, which choro invented for exactly this, keeps the bass moving: by fifths in the chains of dominants, by semitones in the diminished chords, by step in the inversions. A choro chart read with attention to the slashes is a bass line, and the harmony is what that line passes through.
C, E, F, F sharp, G, G, C: the bass of a choro cadence, with the inversion of C, the fourth degree, the passing diminished and the dominant.
Modulation between sections
Classical choro has three parts — A, B and C — in the rondo form A B A C A, and each part usually modulates to a neighbouring key and returns: the second to the relative or the dominant, the third to the subdominant or the parallel. The modulation is confirmed by a cadence in the new key, and the return to A is always in the original.
Here the first part in C and the start of the second in A minor, the relative, with its dominant, E seventh, confirming the new centre. Memorising the form — A in C, B in the relative, C in the subdominant — is worth more than memorising the chords: with the form and the vocabulary, the chords follow.
What to take from here
Choro harmony is functional harmony taken to its limit: every arrival prepared by a dominant, dominants chained by fifths, passing diminished chords linking neighbouring degrees, a bass walking all the time, and modulation to a neighbouring key each section, returning to the original. Whoever knows the form and the vocabulary reads a choro chart as a sentence, and whoever plays choro learns harmony without noticing.