Round or slightly curved sunglasses frames soften a square jaw's angularity, while sharply geometric frame shapes tend to double down on a look that's already strong-jawed.
The Styling Principle
Soften the jaw's hard corner and add movement at the temples and chin. Rounded shapes — in a haircut's ends, in frame lenses, in a beard's edge — counter the squareness without erasing the jaw's natural strength, which most square-faced people are better served by softening than hiding. Round or slightly curved sunglasses frames soften a square jaw's angularity, while sharply geometric frame shapes tend to double down on a look that's already strong-jawed.
Sizing
As with prescription frames, sunglasses width should roughly match your face's widest measurement — for a square face that means sizing relative to forehead or jaw.