, ▁while ▁a ▁cross - c ousin ▁is ▁from ▁a ▁parent ' s ▁opposite - sex ▁sib ling . ▁Thus , ▁a ▁parallel ▁cousin ▁is ▁the ▁child ▁of ▁the ▁father ' s ▁brother ▁( pat ernal ▁uncle ' s ▁child ) ▁or ▁of ▁the ▁mother ' s ▁sister ▁( mat ernal ▁a unt ' s ▁child ), ▁while ▁a ▁cross - c ousin ▁is ▁the ▁child ▁of ▁the ▁mother ' s ▁brother ▁( mat ernal ▁uncle ' s ▁child ) ▁or ▁of ▁the ▁father ' s ▁sister ▁( pat ernal ▁a unt ' s ▁child ). ▁Where ▁there ▁are ▁un il ine al ▁descent ▁groups ▁in ▁a ▁society ▁( i . e . ▁mat ril ine al ▁and / or ▁pat ril ine al ), ▁one ' s ▁parallel ▁c ous ins ▁on ▁one ▁or ▁both ▁sides ▁will ▁belong ▁to ▁one ' s ▁own ▁descent ▁group , ▁while ▁cross - c ous ins ▁will ▁not ▁( ass uming ▁descent ▁group ▁ex og amy ). ▁ ▁Ro le ▁▁ ▁The ▁role ▁of ▁cross - c ous ins ▁is ▁especially ▁important ▁in ▁some ▁cult ures . ▁For ▁example , ▁marriage ▁is ▁promoted ▁between ▁them ▁in ▁the ▁ ▁I ro qu ois ▁system . ▁Par allel ▁c ous ins ▁are ▁occasionally ▁the ▁subject ▁of ▁promoted ▁marriage , ▁such ▁as ▁the ▁prefer ential ▁marriage ▁of ▁a ▁male ▁to ▁his ▁father ' s ▁brother ' s ▁daughter , ▁common ▁among ▁some ▁past oral ▁pe op les . ▁Such ▁a ▁marriage ▁helps ▁keep ▁property ▁within ▁a ▁line age .