“Butter, bread, and green cheese: whoever cannot say that is not a true Frisian.”
Pier Gerlofs Donia (1480–1520) Frisian warrior, pirate, and rebel
Quoted in: The Linguist: Journal of the Institute of Linguists. Volumes 42-43, The Institute, 2003. p. 192
According to legend, Pier forced his captives to repeat this shibboleth to distinguish Frisians from Dutch and Low Germans.