Ludwig van Beethoven's opus 18, published in 1801 by T. Mollo et Comp in Vienna in two books of three quartets each, consisted of his first six string quartets. They were composed between 1798 and 1800 to fulfill a commission for Prince Joseph Franz Maximilian Lobkowitz, who was the employer of Beethoven's friend, the violinist Karl Amenda. They are thought to demonstrate his total mastery of the classical string quartet as developed by Joseph Haydn and Wolfgang Mozart. The order of publication (numbering within the opus) does not correspond to the order of composition. Beethoven composed these quartets in the sequence 3, 1, 2, 5, 4, 6. See:
Beethoven in a letter to Hofmeister in Leipzig (April 8, 1802) refers to the Mollo edition of nos. 4-6 as filled with errors - "has again let us say filled with faults and errata, great and small" and Kerman makes a similar comment, leaving one to conclude that the poor Mollo edition of nos.4-6 - which incited at least strong private protests from the composer - may also at the same time be the best existing primary source for those three works, unless manuscripts or sketches for them have been discovered. This applies only to quartets 4, 5 and 6; the situation for the first quartet in F, especially, is different, since an entire earlier version is preserved, has been published and even recorded.