guitar vs zither

guitar

noun
  • A stringed musical instrument, of European origin, usually with a fretted fingerboard and six strings, played with the fingers or a plectrum (guitar pick). 

  • Any type of musical instrument of the lute family, characterized by a flat back, along with a neck whose upper surface is in the same plane as the soundboard, with strings along the neck and parallel to the soundboard. 

verb
  • To play the guitar. 

zither

noun
  • A musical instrument consisting of a flat sounding box with numerous strings placed on a horizontal surface, played with a plectrum or fingertips. 

  • Related or similar instruments in other cultures, such as the Chinese guqin or Norwegian harpeleik; especially any chordophone without a neck, and with strings that pass over the body. 

verb
  • To make a sound similar to that made by a zither; to move while making such a sound. 

  • To play a zither. 

How often have the words guitar and zither occurred in a corpus of books? (source: Google Ngram Viewer )