The history of fretless zithers is replete with inventions intended to ease the production of chords, first patented in one country and then claimed by someone else shortly thereafter, in a patent issued in another country. When the dates are close enough, it can be difficult to determine who should be credited with the actual innovation. Similarities do sometimes appear to be coincidental but plagiarism was common enough.
The assessment of such situations can be made more difficult by the widespread practice of marking an instrument with the date and number of the first patent issued for it, even if the model at hand incorporates details that do not figure in that patent. One frequently cited example is the way Charles F. Zimmermann applied the number and date of a well-known patent US257808 issued to him in the United States on 9 May 1882.
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