Piano sonatas (Beethoven)
Ludwig van Beethoven wrote his 32 piano sonatas between 1795 and 1822. Although originally not intended to be a meaningful whole, as a set they compose one of the most important collections of works in the history of music. Hans von Bülow called them "The New Testament" of the piano literature (Johann Sebastian Bach's "The Well-Tempered Clavier" being "The Old Testament").
Opus number
To indicate the specific place of a given work within a music catalogue, the opus number is paired with a cardinal number; for example, Beethoven's Piano Sonata No. 14 in C-sharp minor (1801) (nicknamed "Moonlight Sonata") is "Opus 27, No. 2", whose work-number identifies it as a companion piece to "Opus 27, No. 1" (Piano Sonata No. 13 in E-flat major, 1800–01), paired in same opus number, with both being subtitled "Sonata quasi una Fantasia", the only two of the kind in all of Beethoven's 32 piano sonatas. Furthermore, the "Piano Sonata, Op. 27 N° 2, in C-sharp minor" is also catalogued as "Sonata No. 14", because it is the fourteenth sonata composed by Ludwig van Beethoven.