The Piano Sonata in B-flat major, K. 498a (Anh. 136), is a piano sonata in four movements. It was first printed in 1798 by P. J. Thonus in Leipzig on behalf of Breitkopf & Härtel and attributed to Wolfgang Amadeus Mozart; an edition printed in c. 1805 already credited it as opus 26 of the Thomascantor August Eberhard Müller (1767–1817).[1] Some publications still attribute it to Mozart, often as Piano Sonata No. 20.[2]