bass viol vs string bass

bass viol and string bass both are nouns.

Word	Noun	Adjective	Verb	Adverb
bass viol	Yes	No	No	No
string bass	Yes	No	No	No

As nouns, bass viol and string bass are synonyms defined as:

bass viol and string bass: largest and lowest member of the violin family

Other synonyms of bass viol include bass fiddle, bull fiddle, contrabass, double bass.

bass viol (noun)	string bass (noun)
viol that is the bass member of the viol family with approximately the range of the cello	largest and lowest member of the violin family
largest and lowest member of the violin family