crwth vs rebec

crwth

noun
  • An archaic stringed instrument associated particularly with Wales, though once played widely in Europe, and characterized by a vaulted back and enough space for the player to stop each of the six strings on the fingerboard. Played variously by plucking or bowing. 

rebec

noun
  • An early three-stringed instrument, somewhat like a simple violin only pear shaped, played with a bow and used in Medieval and the early Renaissance eras. 

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