Michael Haydn's requiem Pro defuncto Archiepiscopo Sigismundo was written in this key.   
Michael Haydn's requiem Pro defuncto Archiepiscopo Sigismundo was written in this key.   It is the key of the first piano sonata of Karol Szymanowski and of the fourth piano sonata by Prokofiev.  It is also the key of the first symphony by Brahms, as well as his Academic Festival Overture, while Mozart used it both for a Fantasia for solo piano and for an incomplete "Great" Mass.  It was used for the Coriolan Overture, the Piano Sonata number 32, the Fifth Symphony, and the Piano Sonata number 8, or "Path?ique," by Beethoven.  FTP, identify this key which has three flats, the relative minor of E-flat major.
Michael Haydn's requiem Pro defuncto Archiepiscopo Sigismundo was written in this key.   
C minor (do not prompt on "C")