cittern vs gittern

cittern

noun
  • A stringed instrument (chordophone), played with a plectrum (a pick), and most commonly possessing four wire strings and chromatic frets. 

gittern

noun
  • A small, quill-plucked, gut-strung musical instrument, most commonly with three to four strings in doubles courses; it is a flat-backed predecessor of the guitar, and it originated around the 13th century, coming to Europe via Moorish Spain. 

verb
  • To play on the gittern. 

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