The written title of parents-in-law is: parents-in-law Father-in-law is usually used by a wife to address her husband's father or grandfather, and can also be used as a title of respect for older people. Mother-in-law is usually used by wives to address their husbands' mothers, or to address older women, including grandmothers (grandmothers), grandmothers (grandmothers) and great-grandmothers.