In 1951, Otto Friedrich Deutsch, in his Thematic Catalogue of Schubert's Works, classified a transparency manuscript containing the composer's notes, labeled "Symphony" and dated in D major, as a possible attempt at a D major symphony, assigning it the number D615. It is noteworthy that the manuscript contains nine movements, most of which are incomplete and in either D major or related keys, which again cast doubt on Deutsch's interpretation. The cover of the transparency manuscript bears the inscription "Two Symphonies in D," suggesting that it refers to several symphonies. For over 25 years, however, Deutsch's classification remained unchallenged.
When the manuscript was examined more closely in 1978, it was discovered that it contained drafts for three different symphonies from the years 1818, 1821, and 1828. In total, Schubert began composing 13 symphonies, of which he completed seven. After finishing his Sixth Symphony, Schubert turned to the famous "Unfinished" Symphony, which, despite its popularity, is not considered a complete symphony. With the "Unfinished," Schubert entered a new, expressive sound world in symphonic music.
Schubert's symphonies are usually performed as numbers 1 through 9. The Tenth Symphony remained unfinished due to Schubert's death. The surviving fragments of these symphonies offer insight into Schubert's experimental phase and his development as a symphonist. Although many works survive only as fragments, interest in Schubert's "Unfinished" Symphony remains strong, as its final form is left to speculation due to the composer's early death.