The baryton is a bowed string instrument in the viol family, in regular use in Europe up until the end of the 18th century. It most likely fell out of favor due to its immense difficulty to play. Its size is comparable to that of a violoncello; it has seven or sometimes six bowed strings of gut, plus from nine to twenty-four sympathetic wire strings (most often twelve). The gut strings are bowed while the wire strings are plucked by the thumb of the performer in order to create a contrasting tonal quality. It is rarely played today.
From my point of view, the most interesting thing about the instrument is its use of those sympathetic strings, since these are what contribute the most to its distinctive sound. (I have previously written about a similar phenomenon in the theorbo. Just as the sympathetic strings of the theorbo distinguish it from the ordinary lute, the baryton is similarly distinguished from most, but not all, other viols.)
The Wikipedia entry is also most informative about Haydn's connection to this instrument:
Of the repertoire for this instrument, the best known works are the 175 compositions written by Joseph Haydn for his patron, Prince Nikolaus Esterházy, who enjoyed playing this instrument. Of these, 126 are trios for viola, cello and baryton. They were written in the earlier part of Haydn's career, from 1766 to 1775.
Of the trios, John Hsu writes, "Throughout the trios, there is a feeling of intimacy. This is the most private of chamber music, written especially in response to the wishes and needs of one person. We can easily imagine the satisfaction and inspiration which Prince Esterházy experienced while playing these trios." Hsu conjectures that when the Prince played baryton trios, the viola part was taken by Haydn, and the cello part by whoever was the cellist in the Prince's orchestra at the time.
The instrument that the Prince used had seven bowed strings, tuned like a bass viola da gamba (to which the sound of the bowed baryton strings is quite comparable); i.e. AA, D, G, c, e, a d'. This consists of a sequence of rising fourths, except for the third between c and e. The ten plucked strings were tuned in a D-major scale, plus the A a fourth below and the E a major second above.
John Hsu estimates that the Prince was probably not a virtuoso on his instrument, judging from the difficulty of Haydn's writing. The composer used only the top five of the seven bowed strings, and seldom required the player to pluck and bow simultaneously. The keys chosen are also the simplest to play in: mostly D major and the neighboring keys of G major and A major.
Of the trios, critic Lucy Robinson has written "Despite the limitations of the combination, Haydn's genius is evident in the kaleidoscopic range of melodic and textural ideas and the witty interplay between instruments."
The entry also includes this photograph of a replica of Prince Esterhazy's baryton:
With all due respect to Robinson's "kaleidoscopic" perceptions, however, I would have to say that 126 is an awful lot of trios for this instrument; and the Brilliant collection is close to comprehensive, apparently omitting only those that were either lost or fragmentary (at least according to the Hoboken catalog). That comprehensiveness requires a commitment of seventeen CDs, which sends me back to my discontent with the string quartets having been short-changed. Faced with this particular "base camp on Mount Haydn," I find myself reminded of a joke my composition teacher used to tell about a one-sentence book report written by a sixth grader:
This book told me more about penguins than I would ever want to know.
If the collection of baryton trios were as diverse as that of Haydn's piano trios, I could see including the whole package; but, as it stands, that package tells us more about what Haydn had to do to keep his patron happy!