Boater

Boater (fr : Canotier) - A flat-topped, hardened straw hat, with a brim. Often worn as part of the uniform of public schools. The name derives from the fact that the hats were worn by some University scholars, who would go boating on the river, but they became more widely popular as adult headgear in Edwardian times.

Smeltings' boys, where Dudley Dursley has arrived, wore flat straw hats called boaters.

Behind the scenes
Perhaps, this hat of muggles-school is chosen as contrast to Pointed hats at Hogwarts School of Witchcraft and Wizardry.

Appearances

 * PS undefined
 * PS F
 * PS G