Schubert’s Arpeggione Sonata was composed in 1824 but only published in 1871, long after the composer’s death in 1828, and almost as long after the principal instrument for which it was written fell out of favor.
The six-stringed arpeggione was a kind of large bowed guitar that enjoyed a brief period of popularity after its invention in 1823 by the Austrian luthier Johann Georg Stauffer (1778-1853). It was fretted and tuned like a guitar, but held between the legs without a floor peg and played with a bow like the cello.
Schubert may have been prompted to write a sonata for the instrument through his acquaintance with the Viennese arpeggione-player Vincenz Schuster. While the instrument still exists, its adepts are few in number and Schubert’s sonata is mostly played nowadays in transcriptions for viola or cello.
Here is Pinchas Zukerman who will show you the lush sounds of this work on the viola.