20, is a single-movement duo for cello and piano[1] written in 1900 by the Finnish composer Jean Sibelius.

In late 1899 and early 1900, a typhus epidemic swept through southern Finland, near Kerava.

[2] In early March, Sibelius composed—purportedly in just three hours—a duo for cello and piano that he named Fantasia (later retitled Malinconia).

The piece received its premiere on 12 March 1900 in Helsinki; the cellist was Georg Schnéevoigt, the dedicatee, accompanied by the pianist Sigrid Sundgren [fi] (the two later married in 1907).

[1] The Danish cellist Louis Jensen [sv] and the Russian-born Danish pianist Galina Werschenska made the world premiere studio recording of Malinonia in 1936 for the His Master's Voice (since re-released by Warner Classics).