Listening lately to the Boccherini guitar quintets; along with, of course, the Albinoni concerti a cinque. Why is five, peculiarly, the number of perfection in music? Not that you can't do with more (six parts is Josquin's and Brahms' perfect summum), or fewer (as, of course, Haydn's quartets exemplify); but it is to note that, in modern music, there is only one perfect string trio: Mozart's, K. 563 (as I recall). And it is generally admitted that the string quartet has an essentially ambiguous tension between homophony and polyphony, which Haydn, of course, does resolve magisterially--but not always, and it took him awhile to arrive at a perfect balance: The fugal last movements of most of the early Haydn quartets is generally admitted (at least I admit it) to be an unsatisfactory tilt in the direction of polyphony at the expense of the intrinsically homophonic character of the quartet. The balance is not really resolved until you add a fifth part, another viola in the Mozart quintets, another cello in the Boccherini and Schubert quintets. This is why I myself, when I turn composer, usually write music in five parts. I can't exactly say what it is about the perfection of "fiveness," but I hear-tell that there were musical theorists of the Italian Renaissance who wrote on the subject--and I am determined to hunt them down.