bass viol vs double bass

bass viol and double bass both are nouns.

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

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

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

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

bass viol (noun)	double 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