For much of the twentieth century, linguists treated whistled languages, systems in which speakers convey words by whistling, as curiosities practiced by a handful of isolated communities. Fieldwork in recent decades, however, has documented such languages on nearly every inhabited continent, from the Canary Islands to the highlands of Mexico. Researchers now argue that whistling is not a separate language at all but a way of transposing an ordinary spoken language into a sound that carries across valleys and dense forest, and they are using whistled speech to study which features of language listeners rely on most.