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").
Viola Sonata (Mendelssohn)
Felix Mendelssohn composed his Viola Sonata in C minor, MWV Q 14, when he was only 15 years old. The autograph score is dated 14 February 1824. The work was not published in Mendelssohn's lifetime - in fact not until 1966 - and it was not assigned an opus number. Although he did reuse one of the themes from the minuet movement in the equivalent movement of his First Symphony.