Beethoven composed very little original music specifically with the flute in mind. Most of the music he wrote that could be played by the instrument was intended either for flute or violin, underlining the domestic, utilitarian character of these chamber works – you could play them on whatever you had lying around the house.
The Serenade in D major for Flute, Violin, and Viola was one of the exceptions, conceived for a flute and a violin, rather than with interchangeable instruments. This emphasis on soprano instruments leaves the viola to provide the bass, giving the Serenade a sunny, relaxed character.
Beethoven composed the Serenade in 1801 to help the recently-established publisher Giovanni Cappi drum up some business. Coming after the First Symphony, the first two Piano Concertos, and the Septet, Op. 20, the Serenade seems almost retrograde in its persistently cheerful disposition and classical scale, which conforms much more to the requirements of 18th-century salon music than to the grander dimensions of the heroic paths Beethoven had already begun to explore in those other works.
The work is in six movements, its layout conceived according to the pattern of the Classical serenade or divertimento of Mozart’s and Haydn’s time. Beethoven’s innovation was to replace the second minuet with a scherzo (the fifth-movement Allegro scherzando e vivace).
Nowhere is the Serenade’s classical character more apparent than in the slow movement, a set of variations on an Andante theme. These variations decorate the theme and play with its instrumentation in a manner entirely characteristic of the Classical style. This is not the Beethoven of the Diabelli Variations, distilling the essence of a theme over the course of variations that deploy every technique at the composer’s disposal, but rather the composer trained at the princely court in Bonn and in late-18th-century Vienna, adept at music that entertains.
— John Mangum