US 257808, Zimmermann, Charles F., "Harp", published 1882-05-09
fretlesszithers.net
Kelly Williams (May 11, 2003). "Background of the Guitar-Zither". The Guitar-Zither Clearinghouse. terminology on the "Chorded zither" (referred Autoharp without trademark infringement with Oscar Schmidt Inc.), "Guitar-Zither" (patented by Fred Menzenhauer in 1894), and "Chord zither" (referred Guitar-Zither, appeared in The Oxford Companion to Musical Instruments, and The New Grove Dictionary of Musical Instruments).
minermusic.com
Gregg Miner and Kelly Williams (July 2011). "Selecting the Term". Fretless Zithers. terminology and taxonomy of the "Fretless Zither" family instruments.
