Joseph Haydn wrote lots of string quartets. He made this combination popular. Mozart and Beethoven also wrote some very great string quartets. The last ones that Beethoven wrote were very difficult to play and to understand, but composers of the 19th century like Schubert, Schumann, Mendelssohn and Brahms were inspired by them. In the twentieth century some of the best string quartets were written by Bartók and Shostakovich. 