In music, Op. 21 stands for Opus number 21. Compositions that are assigned this number include:
Barber – Capricorn Concerto Beethoven – Symphony No. 1 Britten – Diversions for Piano Left Hand and Orchestra Chopin – Piano Concerto No. 2 Dvořák – Piano Trio No. 1 Enescu – Symphony No. 3 Graham – Episodes Lalo – Symphonie espagnole Larionov – Chout Oswald – Cello Sonata No. 1 Ries – Cello Sonata No. 3 Sarasate – Spanish Dances, Book I Schoenberg – Pierrot lunaire Schumann – Novelletten Sibelius – "Hymn", song for male choir (1896, revised 1898) Villiers – Elegiac Ode Weill – Der Zar lässt sich photographieren