What does it mean to say someone is cool? Sounds like a simple question, doesn’t it? Everyone knows it when they see it, but defining what exactly makes someone cool is trickier than it seems. Is it the aloof restraint Miles Davis maintained while belting out brilliant tunes on the trumpet? Or the boundary-pushing spectacles and outfits Madonna put on? Maybe it’s the raw emotion Johnny Cash poured into his lyrics and performances?

When Joel Dinerstein and Frank H. Goodyear III curated American Cool, an exhibit that’s up now at the National Portrait Gallery, they tried to identify exactly what makes for a legacy of cool.

Dinerstein and Goodyear traced the evolution of cool, from its birth