The four early, consecutively numbered piano sonatas that we hear on this evening’s program were composed over a period of less than two years in 1800 and 1801, toward the end of what Beethoven’s biographer Lewis Lockwood calls his “first maturity.” Already an acknowledged master, the composer had an impressive clutch of masterpieces to his credit, among them 11 piano sonatas, three piano concertos, six string quartets, and a symphony. With the formally innovative Piano Sonata in A-flat Major, Op. 26, Beethoven boldly struck off in a new direction that would soon lead to the even more radically unconventional musical language and structure of the two fantasy-like Op. 27 piano sonatas. The comparatively conservative Piano Sonata in D Major, Op. 28, is largely suffused with a warmth and rustic gaiety that prompted Beethoven’s publisher to dub it the “Pastoral” Piano Sonata. According to his pupil Carl Czerny, however, the composer was partial to the darker strains of the slow movement.