Twins are regularly reported to invent languages of their own, unintelligible to others. These languages are known as autonomous languages, cryptophasia or idioglossia. Despite current belief, this is not a rare phenomenon. Autonomous languages exist in about 40% of all twins, but often disappear soon. In this study, nine autonomous languages are compared: the circumstances in which they emerge, how these languages relate to the parents' language (the model language) and how they are structured. The prototypical situation is one in which two or more close siblings (not necessarily twins) grow up closely together during the language acquisition period. If an adult model language is frequently absent, the children use each other as a model and acquire the language imperfectly. The language may stabilise at that level. If a model is completely absent, the children probably do not create a language. In all cases known, the language consists of onomatopoeic expressions, some invented words, but for the greatest part of words from the adult language adopted to the constrained phonological possibilities of young children. These words being hardly recognizable, the language may turn out to be completely unintelligible to speakers of the model languages, but they resemble each other in that they lack morphology and that word order is based on pragmatic principles such as saliency and the semantic scope of words. Neither the structure of the languages nor its emergence can be explained by other than situational factors.