zither vs zithern

zither and zithern both are nouns.

Word	Noun	Adjective	Verb	Adverb
zither	Yes	No	No	No
zithern	Yes	No	No	No

As nouns, zither and zithern are synonyms defined as:

zither and zithern: a musical stringed instrument with strings stretched over a flat sounding board; it is laid flat and played with a plectrum and with fingers

Other synonyms of zither include cither.

zither (noun)	zithern (noun)
a musical stringed instrument with strings stretched over a flat sounding board; it is laid flat and played with a plectrum and with fingers	a musical stringed instrument with strings stretched over a flat sounding board; it is laid flat and played with a plectrum and with fingers