String Quartet No. 1 (Mendelssohn)  The String Quartet No. 1 in E flat major, Op. 12, was composed by Felix Mendelssohn in 1829, completed in London on September 14 (though begun in Berlin) and possibly dedicated to Betty Pistor, a neighbor and the daughter of a Berlin astronomer. Like all of Mendelssohn's string quartets, this work has four movements:   A typical performance lasts just under 25 minutes.