A linguist figured out New York's class system by asking for the fourth floor

William Labov sent researchers into three New York department stores of different status and asked staff where to find something on the 'fourth floor', a phrase with two chances to pronounce r. The fanciest store dropped fewer r's than the cheap one, showing that a single sound can quietly encode social class.

— William Labov, The Social Stratification of English in New York City — Center for Applied Linguistics, 1966