I listen a lot to Haydn quartets when working... easy yet stimulating to listen too.At last, I want to put in a musical plug for those wonderful Haydn's string quartets op. 20, which lately I have mentioned repeatedly when monitoring system improvements. The opus 20 number is misleading; Haydn wrote those quartets when he was already 40 years old and had written a ton of music before these works (including 40+ symphonies or so).
The Wikipedia page reads:
The six string quartets opus 20 by Joseph Haydn are among the works that earned Haydn the sobriquet "the father of the string quartet". The quartets are considered a milestone in the history of composition; in them, Haydn develops compositional techniques that were to define the medium for the next 200 years.
[...] When Haydn published his opus 33 quartets, ten years after the opus 20, he wrote that they were composed in "an entirely new and particular manner". But, if the opus 33 was the culmination of a process, opus 20 was the proving ground. In this set of quartets, Haydn defined the nature of the string quartet – the special interplay of instruments that Goethe called "four rational people conversing". Many of the compositional techniques used by composers of string quartets to the present day were tried out and perfected in these works.
"This cannot be overstated," writes Ron Drummond; "the six string quartets of Opus 20 are as important in the history of music, and had as radically a transforming effect on the very field of musical possibility itself, as Beethoven's Third Symphony would 33 years later". And Sir Donald Tovey writes of the quartets, "Every page of the six quartets of op. 20 is of historic and aesthetic importance... there is perhaps no single or sextuple opus in the history of instrumental music which has achieved so much".
Here are some of the innovations of the quartets:
Equality of voices
Prior to opus 20, the first violin, or, sometimes, the two violins, dominated the quartet. The melody was carried by the leader, with the lower voices (viola and cello) accompanying. In opus 20, Haydn gives each instrument, and particularly the cello, its own voice. An outstanding example of this is the second quartet in C major. The quartet opens with a cello solo, accompanied by the viola and second violin. This was virtually unheard of in Haydn's time. Another example is in the slow movement of the fourth quartet, in D major. This movement is a set of variations, written in D minor; the first variation is a duet between viola and second violin, and the third variation is a solo for cello.
Here the Wikipedia page gives a note example from Op.20/4, showing interplay between the first violin and cello:
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This is from the first movement of op. 20/4, about 50 seconds in. Here is an example of the music on YouTube, beautifully played by the Attacca Quartet (they also play the album release of "Orange" by Caroline Shaw):
The Wikipedia page continues with a list of other innovations that each are discussed:
Depth of expression
Length and symmetry of phrases
Use of counterpoint
The entire page is a worthwhile read.