'''Symphony No. 91''' in E-flat major, Hoboken 1/91, is written by Joseph Haydn. It was completed in 1788 as part of a three-symphony commission by Comte d'Ogny for the Concerts de la Loge Olympique.Brown, A. Peter, ''The Symphonic Repertoire'' (Volume 2). Indiana University Press (ISBN 025333487X), pp. 232-233 (2002). It is occasionally referred to as ''The Letter T'' referring to an older method of cataloguing Haydn's symphonic output.
The autograph manuscript bears a dedication to D'Ogny, but Haydn also gave Prince Krafft Ernst von Oettingen-Wallerstein a copy as if it were an original.p. 351, Heartz (2009) Daniel. New York. ''Mozart, Haydn and Early Beethoven: 1781 -- 1802'' W. W. Norton & Co.