In the United Kingdom all single first cousins are treated equally and no distinction is made between the gender relationships among their parents. In other countries and cultures different names are sometimes used for the four different types. The 2003 edition of the Encyclopaedia Britannica gives the following nomenclature, as used by anthropologists, to distinguish between them:

"Cousins on the father's side are referred to as patrilateral and those on the mother's side as matrilateral. A first cousin who is the child of a mother's brother or a father's sister is called a cross-cousin and one who is the child of a mother's sister or a father's brother is an ortho-cousin (sometimes called a parallel cousin).''
Thus, the names of the four types are:

Father's brother's child


Patrilateral ortho-cousin

Father's sister's child


Patrilateral cross-cousin

Mother's sister's child


Matrilateral ortho-cousin

Mother's brother's child


Matrilateral cross-cousin

As stated, some societies and languages have different local names for each category and treat them differently from the point of view of perceived incest and preferred marriages. Marriages between ortho-cousins in some countries fall foul of incest taboos. For example, in traditional Chinese society a man may marry his mother's brother's daughter but he may not marry any cousin with his own surname such as his father's brother's daughter.

Genetically, there should be no difference between the four types except when harmful recessive sex-linked genes are present. However, since both sexes are present in all four types of cousins and in their parents and children, no consistant patterns of sex-linked inheritance would be apparent apart from the inbreeding effect. Although the differences are small, a genetic analysis of this effect, shows that the children of patrilateral ortho-cousins suffer the least from the harmful effects of sex-linked genes and those from matrilateral ortho-cousins the most. It is likely, therefore, that the real reason for discrimination is religious, socio-economic or traditional. The results of matings involving sex-linked genes, without inbreeding are shown here.