The 'Arpeggione' was a guitar-like instrument tuned in the same way as the guitar, but held between the knees and played with a bow. The instrument possessed a warm quality and it was this attribute which Schubert exploited in the sonata which he wrote for it in 1824.
Schubert's manuscript on which this edition is based contains an alternative violin part which dates from the same period. To avoid an excess of leger lines and clef changes Schubert wrote the arpeggioned part almost exclusively in the treble clef which the performer was expected to read an octave lower than written. The work was edited for the viola when it came into its own as a solo instrument around the turn of the century.