Sonate für Pianoforte Nr. 12 in A-flat major, Op. 26 by Ludwig van Beethoven: The main theme of Schubert's Impromptu in A♭ major, Op. 142 No. 2 is strikingly similar to the theme in the first movement of Beethoven's sonata. The four-bar phrases that open these pieces are almost identical in most musical aspects: key, harmony, voicing, register, and basic as well as harmonic rhythm.