In Dark Tongues, Daniel Heller-Roazen offers a sustained exploration of a perplexing fact that has never received the attention it deserves. Wherever human beings share a language, they also strive to make from it something new: a cryptic idiom that will allow them to communicate in secrecy. Secret languages may be playful or serious, as apparently impenetrable as a foreign tongue, or only slightly different from the languages from which they spring.

The first recorded jargons date to the time of the Renaissance in Europe. A varied cast of characters -- lawyers, grammarians, and theologians -- denounced these new forms of speech, arguing that they served a single and illegitimate end: crime, plotted in tongues that honest people could not understand. Before this, in epochs and regions as diverse as archaic Greece and Rome, medieval Provence and Scandinavia, singers and scribes also invented opaque varieties of speech, not to defraud but to reveal and record a divine thing: the language of the gods, which poets and priests alone were said to know.

Dark Tongues moves among these hermetic artificial tongues, exploring phenomena as diverse as criminal jargons and divine speech, Saussure's and Tristan Tzara's work of anagrams, Jakobson's theory of subliminal poetic patterning, and the secret writing systems of the Biblical copyists and Druids. In its eleven succinct chapters, Dark Tongues advances a single thesis: that such willfully obscure languages all rest on poetic techniques, which work to play sound and sense against each other.