double bass vs bass viol

double bass and bass viol both are nouns.

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

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

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

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

double bass (noun)	bass viol (noun)
largest and lowest member of the violin family	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