In October 1937, the president of the Dominican Republic, Rafael Trujillo, devised a simple way to identify the Haitian immigrants living along the border of his country. Dominican soldiers would hold up a sprig of parsley—perejil in Spanish—and ask people to identify it. Those who spoke Spanish would pronounce the word's central "r" with that language's characteristic trill; the Haitians, on the other hand, would bury the "r" sound in the throaty way of the French. To be on the receiving end of the parsley test would be to seal, either way, one's fate: The Spanish-speaking Dominicans were left to live, and the Haitians were slaughtered. It was a state-sponsored genocide that would be remembered, in one of history's greatest understatements, as the Parsley Massacre.

Today, thankfully, the stakes of the shibboleth—the term gets its name from the Biblical story—tend not to involve such horrific matters of life and death. On the contrary, they tend to involve matters that don't much matter at all. To a large extent, modern-day shibboleths are status signifiers, the kind of loaded terms that reveal their utterers to be on a single side of a stubbornly binary line. They are not mistakes ("noo-cular" instead of "nuclear," "mis-chee-vee-ous" instead of "mischievous") so much as they are keys: They afford a kind of aural entry into arbitrary echelons. You know you've made it, for better or for worse, when you know that it's pronounced pee-kuh-TEE.