gusle vs viola

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. 

viola

noun
  • A stringed instrument of the violin family, somewhat larger than a violin, played under the chin, and having a deeper tone. 

  • A 10-string steel-string acoustic guitar, used in Brazilian folk music. 

  • A person who plays the viola. 

  • An organ stop having a similar tone. 

  • A berimbau viola, the smallest member of the berimbau used in capoeira music. 

  • Any of several flowering plants, of the genus Viola, including the violets and pansies. 

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