100 portraits of icons who define 'American cool'
6 February 2014 Last updated at 00:42 GMT
The word "cool" is a product of the 1940s African American jazz culture and is now widely used all over the world. But one person's cool may not be another's.
The National Portrait Gallery in Washington DC has taken the brave step of selecting 100 people whose signature style and originality have left a lasting impression on American culture - and in the view of the curators, have earned the title "cool".
American Cool is not intended to be an exhaustive list. It includes Jimi Hendrix, Johnny Depp and Madonna. Hollywood heartthrobs rub shoulders with sports heroes, and some of the photographs are simply classics from their era.
Jane O'Brien spoke to exhibition curator Joel Dinerstein, a scholar of cool from Tulane University who contributed to an accompanying book, to find out what it takes.
Produced by the BBC's Franz Strasser
Picture This is a series of video features published every Thursday on the BBC News website which illustrate interviews with authors about their new books.