bass viol vs string bass

bass viol

noun
  • A fretted, bowed, stringed musical instrument from the viola da gamba family. It occupies the bass range and is the lowest instrument in the family. Similar to (but smaller than) a cello. 

string bass

noun
  • Synonym of double bass (“lowest-pitched of instruments of violin family”). 

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