Most descriptive monographs on primitive tribes contain lists of the words with which the natives designate their relatives by blood and marriage. The reason is far from obvious. Why should not this topic be left in the hands of a linguist-lexicographer? It is true that primitive usage in this regard is very quaint from our point of view, but so are primitive conceptions on a variety of subjects that likewise find expression in speech. The refinement of spatial distinctions in North American languages, the classification of colors or animals or other groups of natural phenomena are of equal intrinsic interest from a psychological point of view. Why, then, single out a particular department of the aboriginal vocabulary in a treatise on culture? The answer is simply this, that kinship terms have a direct relation to cultural data. The very fact that primitive tribes frequently use terms of kinship as words of address where we should substitute personal names is a social practice of ethnological interest. But the essential point is that the terms used are often very definitely correlated with specific social usages. Generally speaking, the use of distinct words for two types of relatives is connected with a real difference in their social relations to the speaker. Thus, a majority of primitive tribes draw no distinction between the father’s sister’s daughter and the mother’s brother’s daughter. But among the Miwok of California, where one of the cousins may be married while the other is within the prohibited degrees, a discrimination is made in language. Again, in many regions of the globe an altogether special bond connects the maternal uncle with the sister’s son, and accordingly we find that he is very often sharply distinguished from the paternal uncle in nomenclature. On the other hand, we can often explain very naturally the use of a single word for two or more relatives whom we designate by as many distinct words. The Vedda of Ceylon, for example, call the man’s father-in-law and maternal uncle by the same term. The reason is that here a man commonly marries his mother’s brother’s daughter; the mother’s brother _is_ his father-in-law, and this identity is expressed in the terminology. A different illustration is supplied by the Crow of Montana, who have one term for the man’s mother-in-law and his wife’s brother’s wife. The simple explanation is that both stand to him in the relationship of mutual avoidance, and it is this social fact that is expressed by the common designation. The same Indians apply the word for ‘father’ in a very inclusive manner, possibly to dozens of individuals; but closer examination shows that all of the people so addressed are entitled to the same kind of treatment by the speaker, to a peculiar form of reverence, and to a preferential rank in the distribution of gifts.