String Quartets and Chamber Music
Haydn's earlier string quartets (divertimenti) each have 5 movements, including 2 minuets. This separates Haydn from Italian composers (such as Luigi Boccherini 1743-1805) whose quartets have 3 movements (slow-fast-slow).
Around 1770 (and op. 20), Haydn switches to begin composing 4-movement quartets.
Haydn's string quartets are "conversations between the players" and were written for private amusement. Beginning in the 1740s, taking off in the 1760s, and perfected in the 1780s, string quartets are the medium by which people learn repertoire (like CD players today, friends would get together and play through the pieces to become familiar with the music).
Early quartets had unequal parts; all the interesting parts were usually in the first violin.
Later quartets incorporated more and more intricacy; other instruments (and more than one) have the melodic line. Haydn becomes more adept at writing quartets.
Mozart hears Haydn's quartets and recognizes them as vastly superior. He studies Haydn's quartets and begins to write his own. However, his response to Haydn's quartets (op. 20) takes him four years. (And Haydn's are still clearly superior to Mozart's early attempts.)
Most of Haydn's chamber music was written for audiences outside of Esterháza. An exception is found in the Baryton trios (published posthumously). These were for Prince Nikolaus's private consumption.
The "Baryton" is an instrument. The bridge is high enough so that the player can bow above the strings and pluck behind them. They have 6 strings.
Haydn wrote many pieces for the baryton because Prince Nikolaus purchased one of these. And, since Haydn was in his employ, he required Haydn to write pieces for the instrument.
In Haydn's pieces, it can be seen that he still favors the violin for the melodic line. It is speculated that this is because Luigi Thomasini was the violinist at Esterházy and was an exceptional musician who could actually play the lines Haydn wrote for him.