Source for notes: The String Quartet, A History by Paul Griffiths, Thames and Hudson 1983.

1. Of all western forms the string quartet is perhaps most capable of reinvention from it's earliest development to the present.
2. This is probably true because it is not only a medium but also a genre and even a form.
3. It's fixed instrumentation of 2 violins, viola and cello give it an identity surpassing easily that of the symphony whose roll call has changed considerably thorough the centuries and even from work to work.
4. One of the most important elements, the voicing, or composing for four voices goes back at least to the end of the 12th century and was standard by the time of Josquin at the end of the 15th century.
5. It is this antiquity of formation that make tracing it's roots so difficult. Because of the popularity of the standard viol consort, established before the Renaissance, the unearthing of a proto-string quartet is always possibly and even likely.
6. The development of basso continuo, however, vexes the theory. In the Baroque the standard chamber music was the Trio Sonata for 2 violins and Continuo. For string quartet to evolve then the addition of a viola was as necessary as the exclusion of the harpsichord.
7. Some consideration may be given to the large amount of music written for performance out of doors in the 18th century - the cassation, the serenade, the divertimento and notturno - (making the use of keyboard inconvenient?) but then why did it not happen earlier?
8. Perhaps the earliest reference to the actual ensemble itself comes from the autobiography of Carl Ditters von Dittersdorf (1739-1799), where he mentions that in 1756 he was rehearsing six (!) new quartets by Richter (Franz Xaver Richter 1709-1789). Richter was a leading composer of the court in Mannheim.
9. At this point we can bring up the name Franz Josef Haydn (1732-1809) who was supposed, until this century, to be the originator, the creator, the animus of the string quartet. The assertion can not really be disproven as there is no reference or definite evidence before his quartets op.1 which were likely written about the same time as von Dittersdorf's reference to Richter's quartets of 1756.
10. The 83 string quartets of Haydn loom so large in a survey of Haydn's compositions that a detailed account of them alone would give a true picture of that composer's style, development, and importance in music history.
11. Some 20 or 30 of these quartets are among the most important in the genre's repertoire.
12. The 12 (some sources say 10) early quartets op.1 and op.2, are distinguished from the rest of his quartets by a five-movement form. Eight of these are symmetrical with slow movements surrounded by minuets with trios and beginning and ending fast movements. There is no trace of continuo and the texture is generally homophonic. The thematic material is based on broken chord patters. Upper voices predominate.
13. By 1769 Haydn's quartets op.9 show a different way and mark another step forward in Haydn's development.. It is the first instance where a true dialogue, wherein themes are transformed by each member, exists, rather than mere imitation. Themes are no longer tied to the tonic triad.
14. The op.17 show virtuosic 1st violin parts because of the resident violinist at Esterhaza (the palace of the Esterhazys) Luigi Tommasini.
15. The op.20 is a milestone. Each quartet (of the 6) is individual. There is more regard for tone color, and a preponderance of minor keys (in keeping with the sturm und drang movement - see Class Updates link). The cello is portrayed more melodically and the #;s 2,5 and 6 have as final movements 4 voice fugues (!).
16. It is in the six quartets op.33 (1781), however, that even the composer states that they were written "in a new and special way".
17. Gone are most of the features that distinguished the previous set (op.20): the fugues, other strict contrapuntal devices, the heavy use of minor mode, etc., to be replaced by Haydn's greatest musical trait and asset, namely wit. Melodies have a greater variety of distinctive intervals and a more elaborate rhythmic scheme. Haydn dissects the music for us, breaking it down into motives, reassembling the fragments with great contrapuntal skill but with no hint of the op.20 approach.
18. Minuets are replace by scherzi and the writing abounds in the meaning of the word: jokes.
19. The primary characteristics of his writing show up in abundance in these quartets: false or 'wrong key' sections, the element of surprise, monothematicism, irregular recapitulation.
20. It was these quartets op.33 that Mozart studied when writing his six masterpieces in the genre and dedicated to Haydn.
21. In actuality, these quartets do not, mark the beginning of Viennese Classicism, if such a date could be fixed. They do, however, give a new character to the quartet idiom. Their approach transformed the genre from a serious conversation to one of profound wit.
22. His last essays in the genre, the Opus 76 were written in 1797 and confront the listener with not only a more 'orchestral' approach in the use of the medium, but with one of his more startling innovations: the finale of the first and third quartets of the Op.76 is in the minor, and only turns to the major during its concluding bars. This radical means of throwing the dramatic weight of the work onto its finale was one that was taken up not only by Beethoven but particularly by Brahms whose knowledge of the Haydn quartets was second to none.
23. The finale of op.76 no.1 also makes the third movement a genuine scherzo - Haydn's first piece of the kind - inspite of the fact that the op.33 quartets, nearly 20 years earlier, had included movements headed by Scherzo or Scherzando, as a substitute for the traditional minuet; but stylistically the pieces in question had been minuets in all but name.
24. Haydn does not use the term scherzo in his op.76: by the time they had appeared, Beethoven's op.1 piano trios had already been widely circulated. The first two of those trios contain a thoroughly characteristic scherzo; and if, as seems likely, they provided the source of Haydn's inspiration, it is understandable that he wanted to avoid direct comparison with his former pupil.