Duke Max' profile in Wikipedia says:
Maximilian Joseph was one of the most prominent promoters of Bavarian folk-music in the 19th century. Under his influence the zither started to be used in court circles and eventually became identified as the national musical instrument of Bavaria. Because of his interest he received the nickname Zither-Maxl. He himself played the zither and also composed music for it.
From a very informative article by Ernst Schusser on the Zither US website:
The association of Duke Maximilian of Bavaria (1808-1888) with the zither was very greatly influenced by his getting to know Johann Petzmayer through two concerts given by the zither virtuoso on the 22nd and 26th of February, 1837, in Bamberg, Germany. Johann Petzmayer was born in the town of Zistersdorf near Vienna, Austria, in 1803. His father owned and operated a tavern in Vienna, where he grew up. In his early youth he learned to play the violin and at 16 years of age, he just happened to start playing the zither, with which he immediately became enamored. Through diligent study, he developed an astounding ability in playing this simple instrument ...Also good information in an article in Traunsteiner Tagblatt/Chiemgau Blätter of Munich, touching on folk zithers in the Bavarian and Austrian Alps and Duke Max' role in popularizing the instrument in upper-crust "Salons und Konzertsäle" in Bavaria:
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At the behest of the Duke, Petzmayer (as musical specialist) assembled other musicians around the Duke, certainly for his, but also for mutual enjoyment. Max himself often played his own compositions as well as folksongs on the zither in this group. The circle of musicians, the various society functions and gatherings, but also quiet and undisturbed moments were opportunities for Duke Max to reach for his zither. The zither played a much larger role in the life of Duke Maximilian than simply a beloved instrument, a sentiment underscored by one of the very few poems written by Max, entitled appropriately enough “My Zither“ ...
[extended quote from poem]
However, it is not only with their zither playing and music making that I wish to draw attention, because through their collaboration, Petzmayer and Duke Maximilian also contributed immeasurably to the development and dissemination of the zither. First of all, was the simplification and specification of the term “Zither“, which started about 1840, the time that Duke Max first took pains to support zither playing. Previously, the term “Zither“ included all the many different historical forms and types of instruments, such as the psalterium, scheitholz, lute, guitar, violine, hammered dulcimer, etc. just as with the zither we have today one can find similar instrument forms and further developments. In Musicology works one often finds the expression “junk“ instrument in relation to the zither of that time. This started to change first through the efforts of Johann Petzmayer around 1825 as he developed into the first zither-virtuoso of the modern zither in the 19th century. This, coupled with the efforts of Duke Maximilian from around 1837, led to the unambiguous meaning of the term.
At that time, the term zither meant a stringed instrument with two play areas, namely, the fretboard, with its changeable tones and the free strings with their permanently fixed tones. “The strings run parallel to and over a narrow box-like resonating body, and are made to ring by a combination of gripping and striking with both hands.“ This zither is represented therefore as a combination of both a “gripped“ and an “ungripped“ instrument, which in some cases is historically accurate. Coming into the 19th century, the zither was thoroughly unfinished and in need of improvement. More than anyone else, it is Petzmayer who we have to thank that this instrument in its unfinished form had already begun to gain some popularity around 1825. This type of instrument is called Petzmayer’s “Plucking Zither“, which is most likely where the term “to pluck the zither“ comes from. Technically the zither is called diatonic. At the most, one could play only three tones, most often G, D, and A. The fretted fingerboard contained just 14 notes. At the beginning of the 19th century there were only three notes on the fingerboard. The tuning of the free strings, used mostly for accompaniment, was without any system, one claimed they belonged to the “good sound of the zither player,“ so that each player developed his own system for stringing the instrument. Most of the early drawings depicting Petzmayer and Duke Maximilian show us this type of zither, which is quite individual and totally without standardization, which soon showed what an impediment it was to further dissemination of the instrument. ...
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Here then we have the two zither styles facing each other. One style, deeply rooted by custom and by their way of life, was considered by the majority of people to belong to the “country folks“ from which only personalities like Josef Wasserburger (1788-1857) “Joey, the Innkeeper from Garching“ stood out. On the other side one could see the extremely useful results of Weigel and the other reformers to lead the zither out of its role as folk instrument and to have it gain musical acceptance. I would place Petzmayer and above all others the late Duke Max who incorporated the folkloric as well as the classical-musical character in the zither. The Duke clearly preferred the folkloric part. This twin track, was under the circumstances in my opinion, especially worthwhile. It offered a great chance of success, led to the zither’s first heyday. As little as Petzmayer forgot about the somewhat lowly origins of the zither, so did his accomplished, masterful style of playing lead him more and more into contact with the upper and noble circles, in fact, all the way to the courts of kings and emperors. This is where the instrument for the first time made the leap from being an everyday common instrument to being an instrument of high society, it became one could say “suitable for the court!“ This development would not have lasted if it had not been for Duke Max making it his cause and strengthening it.
Zur Liedbegleitung hatten die einfachen Zithern auf dem Land vor allem im bayerisch-österreichischen Alpenraum und auch im Alpenvorland bereits sehr früh eine breite Verbreitung gefunden und auch in München war das Instrument beliebt. Bereits von 1643, also fünf Jahre nachdem die Zunft der Münchner Stadtmusikanten gegründet worden war, wird überliefert, dass ein Viertel der Musikanten »Zitherschlager« waren.Works Cited
Die wahre Blüte des Zitherspiels in München kam jedoch erst im 19. Jahrhundert, als der Instrumentenbau bedeutende Fortschritte machte. Damals erhielt die Zither mehr Saiten und es setzte sich eine neue, Münchner Stimmweise durch. Den größten Anteil an der Zitherbewegung hatte jedoch um die Mitte des 19. Jahrhunderts der Virtuose Johann Petzmayer und sein begnadeter Schüler und Protegé Herzog Max in Bayern, der wegen seiner Zitherleidenschaft auch »Zither-Maxl« genannt wurde. Ihm ist es zu verdanken, dass das »Lumpeninstrument« auch in die höfischen Kreise und damit in die soziale Oberschicht Einzug fand. Diesen beiden Virtuosen ist es zu verdanken, dass die Zither mehr und mehr auch Solostimmen spielen konnte. Gleichzeitig eroberte die Zither die Salons und Konzertsäle. Dort wurden vor allem auch auf die Zither umgeschriebene Kammermusik vorgetragen. Die Bemühungen des Herzogs, der Zither Eingang in die Gesellschaft zu verschaffen, waren von großem Erfolg gekrönt. Bald galt es als modern, in städtischen, bürgerlichen oder adeligen Haus Zither zu spielen. Ganze Heerscharen von sogenannten höheren Töchtern erlernten dieses Instrument, zumal es auch wesentlich billiger als ein sonst obligates Klavier war. Im ersten Weltkrieg hatten bei den Instrumentenbauern zusammenlegbare Zither, »die dem lieben Ehegatten ins Feld nachgeschickt werden« konnten, eine große Bedeutung.
“Duke Maximilian Joseph in Bavaria.” Wikipedia. http://en.wikipedia.org/wiki/Duke_Maximilian_Joseph_in_Bavaria. Picture available in Wikimedia Commons.
Ernst Schusser "Zithermaxl." Trans. William F. Kolb. Zither US. http://www.zither.us/zithermaxl
"Die Zither is a Zauberin: München kann als die bedeutenste Zitherstadt der Welt angesehen werden." Traunsteiner Tagblatt/Chimemgau Blätter 17 Jan. 2004