This thesis analyzes the first movement of Franz Schubert's Piano Sonata in B-flat major, D. 960, with special attention to issues of sonata form. A Schenkerian viewpoint is adopted, and that choice is justified by Schenkerian analysis' ability to attend to the most important rhetorical moments of sonata form. The author's Schenkerian interpretation is compared with several algebraic or 'neo-Riemannian' analyses, and the advantages of the Schenkerian analysis are clarified. Passages from several other works are invoked to illustrate the role that third-related harmonies play in those works as well as in the Schubert sonata.