Brahms wrote and premiered his Variations on a Theme by Joseph Haydn in 1873. The real author of the original theme (called St. Anthony's Chorale) is still in dispute, though some scholars believe it was written by Ignaz Pleyel, who was Haydn's pupil. In any case, to avoid using Haydn's name, the piece is nowadays referred to as the Saint Anthony Variations.
The first version finished by Brahms was for two pianos, although this was only published after the orchestral version, which went on to become much more popular. Because of that, the two pianos version is called Op. 56b and the orchestral Op. 56a. The score calls for 1 piccolo, 2 flutes, 2 oboes, 2 clarinets, 2 bassoons, contrabassoon, 4 horns (2 in E flat, 2 in B flat), 2 trumpets, triangle, timpani, and strings.