Cantus Firmus

Bach set the same Lutheran hymn tunes over and over. A melody that opens a cantata in 1724 comes back as a four-part harmonisation, and again under an organist's hands, and again in a Passion. Those borrowed tunes are the cantus firmus — the fixed song a composer builds around — and they are the hidden wiring of the catalogue. This is a map of them, made for the phone. Open a work and you get its movements, with type, key and scoring. Where a movement sets a chorale melody, a chain-link icon says how many other works set the same one. Tap it and there they are, grouped by the tune they share — so a connection reads as a fact about the music rather than a line on a graph. THE CATALOGUE • 554 works — cantatas, motets, the two Passions, the Christmas Oratorio, four-part chorales, organ chorales, concertos • 1,940 movements • 209 chorale melodies, each with its Zahn number and every place…