The function of language is communication, but accurate communication is impossible unless the language is mutually understood. In an extensive review of the literature of conjoined twins, considerable difficulty was encountered in interpreting the variations in nomenclature--the designation of the different types of union as well as the identification of the individual twins and their anatomic relationship to one another. For example, the term thoracopagus often includes twins with either one or two hearts; the former rarely (if ever) is capable of surviving, but the latter often do survive. Labeling them as thoracopagus and omphalopagus, respectively, will permit more meaningful comparison between the two types. A simple, logical system of terminology is proposed.