The iconic existentialist black turtleneck fashion came after the war, not with it.

Evidence

To go with the jazz, blues and ragtime after the war, people sought out American clothes, readily available in flea markets; there was a particular craze for plaid shirts and jackets. If your twenty-first-century time machine could take you back to a Parisian jazz club immediately after the war, you would not find yourself in a sea of existentialist black; you would be more likely to think you’d walked into a lumberjacks’ hoedown. An impression of the effect can be had from Jacques Becker’s film Rendezvous de juillet, released in 1949, which features an exuberant dance scene in the Lorientais club: as Claude Luter’s band plays on the cramped stage, the check-shirted crowd leaps around on the dance floor. The sleek black turtleneck arrived afterwards — and when Americans in turn adopted that fashion, few realised they were returning a sartorial compliment. — Sarah Bakewell, At the Existentialist Café, 537

Categorization

• CANDY: A fun party fact: the classic existentialist look actually started as plaid lumberjack shirts, and the turtleneck was a later import.
• SEED: Essay angle: how a movement’s external visual branding (black turtlenecks) gets invented retrospectively, long after the movement’s actual lived aesthetic.