Speakers of Dyirbal in North Queensland, Australia, have an everyday language style which has been well described. They also have an avoidance style, called Jalnguy, which must be used in the presence of certain ‘tabooed kin’ such as the mother-in-law. Jalnguy has the same grammar and phonology as the everyday style, but the vocabularies are entirely different. Jalnguy has only about one-sixth as many lexemes as the everyday style, with various techniques used to create a Jalnguy correspondent for each everyday style word. The underlying semantic system of Dyirbal is thus realised at two levels of generality and is revealed through detailed study of the correspondences between them. This monograph is the first comprehensive study of this in-law avoidance language, providing fascinating new insights into register variation and the complexities of kinship in Australian languages.