Autograph first page of a fugue from Bach’s Well-Tempered Clavier. Via Wikimedia Commons (public domain).
Theme and variations seems the easiest form to listen to: there is a melody, and then the same melody dressed up in ten ways. In reality it is the form in which the listener gets lost first, and the reason is a misunderstanding: we try to follow the melody. Mistake. In most great sets of variations, the melody is the first thing the composer throws away.
What remains, and holds everything up, is the skeleton beneath: the harmonic progression and the bass. Bach’s Goldberg Variations are the perfect example: of the opening aria, so famous, hardly a note survives in the thirty variations. What survives is the bass, the same thirty-two bars of foundation, on which Bach builds thirty different buildings. The right way to listen to them is the same one we recommend for the fugue: ear low, where the structure lives, and let the acrobatics above arrive on their own.
Once you grasp the mechanism, the game becomes recognising the moves, which are surprisingly few and recur from Byrd to Brahms. The variation that speeds up, where the theme is minced into fast notes. The one in the minor, always at least one, the moment when the party clouds over. The slow, ornate one, the opera aria in disguise. The canon or fugato, when the composer shows off the muscles of counterpoint. And the finale, which almost always either returns to the bare theme, as in the Goldbergs, with a leave-taking effect that wrings the heart, or explodes in a whirl that gives the theme the merriest funeral possible.
Then there is the summit of the form, where variation becomes philosophy: starting from deliberately poor material and drawing a world out of it. Diabelli sent Austrian composers a cheap little waltz, asking for one variation each; Beethoven, who called it a cobbler’s patch, wrote thirty-three variations on it all by himself, the greatest piano work of the nineteenth century. The message, valid outside music too, is that the theme counts for almost nothing: what counts is what you see in it. That is why the form has never died, from the Baroque ground bass to the chord changes jazz improvises on, to the producer who samples four bars and builds a track on them: they are all variations, and anyone with an ear trained on the skeleton recognises them at once. The theme is the pretext. The variation is the thought.