Dark Tongues constitutes 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, built from the grammar that they know, which will allow them to communicate in secrecy. Such hidden languages come in many shapes. They may be playful or serious, children's games or adults' work. They may be as impenetrable as foreign tongues, or slightly different from the idioms from which they spring, or barely perceptible, their existence being the...