These are called exogamous groups or clans, and their name among the higher castes is _gotra_.

The theory is that all persons belonging to the same _gotra_ are descended from the same male ancestor, and so related.

The relationship in the _gotra_ now only goes by the father's side; when a woman marries she is taken into the clan of her husband and her children belong to it.

Marriage is not allowed within the clan and in the course of a few generations the marriage of persons related through males or agnates is prohibited within a very wide circle.

But on the mother's side the _gotra_ does not serve as a bar to marriage and the union of first cousins would be possible, other than the children of two brothers.