Of course, one use double quotes when you want to interpolate, and single quotes when you don't. No question about that. The topic however is the large set of strings where "are we interpolating or not" isn't a sensible question. Are you interpolating in "Hello, world"? Perhaps you are, it just happens the set of variables you are interpolating happen to be zero. For me, double quotes also mean "I'm interpolating". But often, I interpolate zero variables.