The guitar zither (also chord zither, fretless zither,, mandolin zither or harp zither) is a musical instrument consisting of a sound-box with two sets of unstopped strings. One set of strings is tuned to the diatonic, chromatic, or partially chromatic scale and the other set is tuned to make the various chords in the principal key of the melody strings.
First patented May 29, 1894 by Friederich Menzenhauer (1858-1937), the guitar zither came into use in the late 19th century and was widely mass-produced in the United States and in Germany by Menzenhauer and later by Oscar Schmidt Inc., the Phonoharp_Company, and others 
The 1902 Sears catalog touted the Deweylin Harp — a type of fretless zither — as "...the wonder of the age," and "...the greatest musical instrument that has ever been placed before the public." These mandolin-guitar-zithers purported to combine three instruments for the price of one.
A form of psaltery and member of the family of chordophones, the guitar zither is closely related to the decades-later Autoharp. It differs from the concert zither in not having a fretboard and from the Autoharp in not having a mechanical process for blocking chords.
Kelly Williams (May 11, 2003). "Background of the Guitar-Zither". The Guitar-Zither Clearinghouse.
terminology on the "guitar-zither" (patented by Menzenhauer), "chord zither" (referred guitar-zither, appeared in The Oxford Companion to Musical Instruments and The New Grove Dictionary of Musical Instruments), and "chorded zither" (referred Autoharp without trademark infringement with Oscar Schmidt International).
Gregg Miner & Kelly Williams (July 2011). "Selecting the Term". Fretless Zithers.
terminology and taxonomy of the "Fretless Zither" family instruments.
- Andreas Michel. "Harp zither". In Deane L. Root (ed.). Grove Music Online. Oxford Music Online. Oxford University Press. (subscription required)
- Guitar-Zither, Specification forming part of Letters Patent No. 520,651, dated May 29, 1894. Application filed April 20, 1893, Serial No.471,147. (Ho model.), United States Patent Office