gusle vs rebec

gusle

noun
  • A single-stringed lute-like stringed instrument with a bowl-shaped body, held vertically in the lap and played a bow, originating among the Slavic peoples in the Balkans, especially in the Dinarides region. 

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 gusle and rebec occurred in a corpus of books? (source: Google Ngram Viewer )