Double-bass and String bass

Double-bass adjective - Pitched an octave below normal bass instrumental or vocal range.

String bass and double-bass are semantically related. In some cases you can use "String bass" instead a noun phrase "Double-bass".

String bass noun - Largest and lowest member of the violin family.

Double-bass and string bass are semantically related. Sometimes you can replace phrase "Double-bass" with "String bass", this phrases are similar.