string bass vs double bass

string bass and double bass both are nouns.

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

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

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

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

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