70 is a composition for piano and cello, written jointly by Frédéric Chopin and Auguste Franchomme.
[1] Chopin had initially been contracted by his publishers to write a work for piano based on Giacomo Meyerbeer's opera Robert le diable.
He had attended a performance and liked the work, but was disinclined to write a "fantasia" (as he put it in a letter to his family) on another composer's music.
However, his friend Auguste Franchomme persuaded him to jointly write a piece for cello and piano, using themes from the opera.
[2] The piece was published under both their names, and was favourably reviewed by Robert Schumann.