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Kinship terminology is the system used in languages to refer to the persons to whom an individual is related through kinship. Different societies classify kinship relations differently and therefore use different systems of kinship terminology; for example, some languages distinguish between consanguine and affinal uncles (i.e. the brothers of one's parents and the husbands of the sisters of one's parents, respectively), whereas others have only one word to refer to both a father and his brothers. Kinship terminologies include the terms of address used in different languages or communities for different relatives and the terms of reference used to identify the relationship of these relatives to ego or to each other.
Anthropologist Lewis Henry Morgan (1818-1881) performed the first survey of kinship terminologies in use around the world. Though much of his work is now considered dated, he argued that kinship terminologies reflect different sets of distinctions. For example, most kinship terminologies distinguish between sexes (the difference between a brother and a sister) and between generations (the difference between a child and a parent). Moreover, he argued, kinship terminologies distinguish between relatives by blood and marriage (although recently some anthropologists have argued that many societies define kinship in terms other than blood).
However, Morgan also observed that different languages (and, by extension, societies) organize these distinctions differently. He proposed to describe kinship terms and terminologies as either descriptive or classificatory. When a descriptive term is used, it can only represent one type of relationship between two people, while a classificatory term represents one of many different types of relationships. For example, the word brother in English-speaking societies indicates a son of the same parent; thus, English-speaking societies use the word brother as a descriptive term. A person's male first cousin could be the mother's brother's son, mother's sister's son, father's brother's son, father's sister's son, and so on; English-speaking societies therefore use the word cousin as a classificatory term.
Morgan discovered that a descriptive term in one society can become a classificatory term in another society. For example, in some societies, one would refer to many different people as "mother" (the woman who gave birth to oneself, as well as her sister and husband's sister, and also one's father's sister). Moreover, some societies do not group together relatives which the English-speaking societies classify together. For example, some languages have no one-word equivalent to cousin, because different terms refer to one's mother's sister's children and to one's father's sister's children.
Armed with these different terms, Morgan identified six basic patterns of kinship terminologies:
Some languages, such as Kannada, Telugu, Tamil, Turkish, Sinhalese, Chinese (see Chinese kinship), Japanese, Korean, Khmer, Vietnamese, Tagalog (Filipino), Hungarian, Bulgarian, Nepalese, and Nahuatl add another dimension to some relations: relative age. Rather than one term for "brother", there exist, for example, different words for "older brother" and "younger brother". In Tamil, an older male sibling is referred to as Annan and a younger male sibling as Thambi, whereas older and younger female siblings are called Akka and Thangai respectively.
Other languages, such as Chiricahua, use the same terms of address for alternating generations. So Chiricahua children (male or female) call their paternal grandmother -ch'iné, and likewise this grandmother will call her son's children -ch'iné. Similar features are seen also in Huichol, some descendant languages of Proto-Austronesian (e.g. Fordata, Kei, and Yamdena), Bislama, and Usarufa. Terms that recognize alternating generations and the prohibition of marriage within one's own set of alternate generation relatives (0, ±2, ±4, etc.) are common in Australian Aboriginal kinship.
The relative age and alternating-generations systems are combined in some languages. For instance, Tagalog borrows the relative age system of the Chinese kinship and follows the generation system of kinship. Philippine kinship distinguishes between generation, age and in some cases, gender.
Floyd Lounsbury described a seventh, Dravidian, type of terminological system that had been conflated with Iroquois in Morgan's typology of kin-term systems because both systems distinguish relatives by marriage from relatives by descent, although both are classificatory categories rather than being based on biological descent. The basic idea is that of applying an even/odd distinction to relatives that takes into account the gender of every linking relative for ego's kin relation to any given person. A MFBD(C), for example, is a mother's father's brother's daughter's child. If each female link (M,D) is assigned a 0 and each male (F,B) a 1, the number of 1s is either even or odd; in this case, even. However, variant criteria exist. In a Dravidian or Tamil system with a patrilineal modulo-2 counting system, marriage is prohibited with this relative, and a marriageable relative must be modulo-2 odd. There exists also a version of this logic with a matrilineal bias. Discoveries of systems that use modulo-2 logic, as in South Asia, Australia, and many other parts of the world, marked a major advance in the understanding of kinship terminologies that differ from kin relations and terminologies employed by Europeans.
The Dravidian or Tamil kinship system involves selective cousinhood. One's father's brother's children and one's mother's sister's children are not cousins but brothers and sisters one step removed. They are considered consanguineous (pangali in Tamil), and marriage with them is strictly forbidden as incestuous. However, one's father's sister's children and one's mother's brother's children are considered cousins and potential mates (muraicherugu in Tamil). Marriages between such cousins are allowed and encouraged. There is a clear distinction between cross cousins, who are one's true cousins and parallel cousins, who are, in fact, siblings.
Like Iroquois people, Dravidians use the same words to refer to their father's sister and mother-in-law (atthai in Tamil and atthe in Kannada) and their mother's brother and father-in-law (maamaa in Tamil and maava in Kannada). In Kannada, distinction between these relationships may be made because sodara is added before atthe and maava to specifically refer to one's father's sister and mother's brother respectively, although this term is not used in direct address. In Tamil, however, only one's mother's brother is captioned with thaai before maamaa because of the honor accorded this relationship.
The genealogical terminology used in many genealogical charts describes relatives of the subject in question. Using the abbreviations below, genealogical relationships may be distinguished by single or compound relationships, such as BC for a brother's children, MBD for a mother's brother's daughter, and so forth.