It wasn't difficult to pick a starting point for a detailed study of the string quartet. Haydn's Op. 20 was the first great set of string quartets and was more responsible than any other set for the status the string quartet enjoys as a major art form worthy of the best efforts of any composer.
My notes which are linked below are intended to point out the many interesting features and relationships that I was able to find. I hope that they show how carefully planned Op. 20 is when taken in the original order as recorded in Haydn's catalog of his works. The reader is encouraged to listen to them all in this sequence!