The five string quartets in this volume are generally designated as Beethoven's ''middle quartets,'' even though stylistically they do not all build a closed unit. More than three years lie between the inception of the three quartets op. 59 (1806) and the quartet op. 74 (1809/10). In the quartets op. 59, Beethoven sprinkled several Russian folksong themes - an homage to Prince Razumovsky, who had commissioned the works.This is why the pieces are often called the ''Russian Quartets.'' The next quartet, op. 74 in E flat major, was also given a nickname: thanks to extended pizzicato passages in the first movement, it is frequently called the ''Harp Quartet''. The f minor Quartet op. 95, in turn, bears in the autograph title ''Quartett serioso.'' All five quartets are mature masterworks which demand high interpretative and technical mastery.