cittern vs psaltery

cittern

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

psaltery

noun
  • A zither-like musical instrument consisting of a soundboard with multiple strings, played by plucking the strings with the fingers or a plectrum. 

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