Both in Polynesia and Melanesia it is not infrequent for the father-in-law to be classed with the father, the mother-in-law with the mother, the brother-in-law with the brother, and the sister-in-law with the sister. The Oceanic terminology of relationship has two features which enable us to study the exact nature of this process in more detail than is possible with our own system. Oceanic languages often distinguish carefully between different kinds of brother- and sister-in-law, and, if it be found that it is only certain kinds of brother- or sister-in-law who are classed with the brother or sister, we may thereby obtain a clue to the nature of the process whereby the classing has come about. Secondly, Oceanic terminology usually distinguishes relationships between men or between women from those between persons of different sex, and there is a feature of the terminology employed when brothers- or sisters-in-law are classed with brothers or sisters in Oceania which throws much light on the process whereby this common nomenclature has come into use. The first point to be noticed in the Oceanic nomenclature of relationship is that not all brothers- and sisters-in-law are classed with brothers and sisters, but only those of different sex. Thus, in Merlav, in the Banks Islands, it is only the wife’s sister and a man’s brother’s wife who are classed with the sister, and the husband’s brother and a woman’s sister’s husband who are classed with the brother, while there are special terms for other categories of relative whom we include under the designations brother- and sister-in-law. Similar conditions are general throughout Melanesia. If, as Professor Kroeber has supposed, the classing of the brother-in-law with the brother be due to the psychological similarity of the relationships, we ought to be able to discover why this similarity should be greater between persons of different sex than between persons of the same sex.