Variations on a Waltz by Diabelli, op. 120

The 33 Variations on a waltz by Anton Diabelli, Op. 120, commonly known as the Diabelli Variations, is a set of variations for the piano written between 1819 and 1823 by Ludwig van Beethoven on a waltz composed by Anton Diabelli. It forms the first part of Diabelli's publication Vaterländischer Künstlerverein, the second part consisting of 50 variations by 50 other composers. It is often considered to be the greatest set of variations for keyboard along with Bach's Goldberg Variations.
The music writer Donald Tovey called it "the greatest set of variations ever written" and pianist Alfred Brendel has described it as "the greatest of all piano works". It also comprises, in the words of Hans von Bülow, "a microcosm of Beethoven's art". In Beethoven: The Last Decade 1817–1827, Martin Cooper writes, "The variety of treatment is almost without parallel, so that the work represents a book of advanced studies in Beethoven's manner of expression and his use of the keyboard, as well as a monumental work in its own right". In his Structural Functions of Harmony, Arnold Schoenberg writes that the Diabelli Variations "in respect of its harmony, deserves to be called the most adventurous work by Beethoven".
Beethoven's approach to the theme is to take some of its smallest elements – the opening turn, the descending fourth and fifth, the repeated notes – and build upon them pieces of great imagination, power and subtlety. Alfred Brendel wrote, "The theme has ceased to reign over its unruly offspring. Rather, the variations decide what the theme may have to offer them. Instead of being confirmed, adorned and glorified, it is improved, parodied, ridiculed, disclaimed, transfigured, mourned, stamped out and finally uplifted".
Beethoven does not seek variety by using key-changes, staying with Diabelli's C major for most of the set: among the first twenty-eight variations, he uses the tonic minor only once, in Variation 9. Then, nearing the conclusion, Beethoven uses C minor for Variations 29–31 and for Variation 32, a triple fugue, he switches to E♭ major. Coming at this late point, after such a long period in C major, the key-change has an increased dramatic effect. At the end of the fugue, a culminating flourish consisting of a diminished seventh arpeggio is followed by a series of quiet chords punctuated by silences. These chords lead back to Diabelli's C major for Variation 33, a closing minuet.