After completing the Ninth Symphony, he devoted his energies largely to composing string quartets, although there are contemporary references to some work on a symphony (e.g. in a Beethoven letter of 18 March 1827); allegedly he played a movement of this piece for his friend Karl Holz, whose description of what he heard matches the material assembled by Cooper. Cooper claimed that he found over fifty separate fragments, which he wove together to form the symphonic movement. Cooper assembled material for a first movement consisting of an Andante in E-flat major enclosing a central Allegro in C minor. Cooper claims to have also found sketches for a Scherzo which are not developed enough to assemble into a performing version.
There are numerous references to this work in Beethoven’s correspondence (originally, he had planned the Ninth Symphony to be entirely instrumental, the Ode to Joy to be a separate cantata, and the Tenth Symphony to conclude with a different vocal work).
Earlier, in 1814-15, Beethoven also began sketches for a 6th piano concerto in D major, Hess 15. (Unlike the fragmentary symphony, the first movement of this concerto was partly written out in full score and a reconstruction by Nicholas Cook has been performed and recorded.)
An imaginary story of the discovery of Beethoven’s 10th symphony has been depicted by Sue Latham in her novel The Haunted House Symphony