Plural of “mother-in-law”

The plural of mother-in-law is mothers-in-law (some dictionaries also accept mother-in-laws).

Why

"Mother-in-law" follows the identical compound structure as "brother-in-law": "mother" is the head noun naming the actual person, and "in-law" is a fixed modifying phrase specifying that the relationship exists through marriage rather than blood. The plural -s attaches to "mother," the true head of the compound, giving "mothers-in-law" rather than "mother-in-laws."

Unlike "brother-in-law," which can describe three distinct relationships, "mother-in-law" describes only one: a spouse's mother, making it grammatically identical in structure but semantically simpler than several of its "in-law" relatives. The term, and the whole "in-law" vocabulary system built around it, entered English family terminology through French-influenced medieval legal and canon-law language, reflecting a period when the Church's rules about marriage and prohibited relationships made a precise vocabulary for relationships "by law" rather than blood increasingly important in everyday legal and social life.

Usage note

"Mothers-in-law" is the standard, correct plural in every formal and careful writing context, including legal, medical, and genealogical documents discussing multiple such relationships at once. "Mother-in-laws" appears in casual speech and informal writing but is considered nonstandard and should generally be avoided outside the most relaxed registers.

The mother-in-law relationship carries an unusually rich cultural and comedic history across many societies, frequently the subject of jokes, folk sayings, and popular stereotypes about friction between a wife and her husband's mother, or occasionally the reverse, a cultural pattern documented across many unrelated languages and traditions, suggesting something close to a universal social dynamic rather than a peculiarity of English-speaking culture alone. Family law documents distinguishing "mothers-in-law" from "stepmothers" or "adoptive mothers" matters in inheritance and custody cases, since each relationship carries different legal weight and different default rights depending on jurisdiction. Some cultures place mothers-in-law in a formal position of household authority, particularly in traditional extended-family living arrangements, while others treat the relationship as largely social and non-hierarchical, a real cultural variation that shapes how the word's connotations differ depending on the community being described.

Common real mistakes

By far the most common slip is "mother-in-laws," tacking the plural onto the very end of the hyphenated string instead of onto "mother," the noun the phrase is actually naming. A rarer hypercorrection, "mothers-in-laws," doubles down by pluralizing both ends of the compound at once, which is just as wrong as the first error, only in the opposite direction.

The wider pattern

"Mother-in-law" is one of the core members of the "in-law" kinship family alongside "brother-in-law," "sister-in-law," "son-in-law," and "daughter-in-law," all sharing the identical noun-plus-modifying-phrase structure and identical pluralization rule: the -s always goes on the blood-relation noun at the front, never on "law" at the end.

Examples

• Both of their mothers-in-law attended the christening.
• She invited her mother-in-law and her sister's mother-in-law to the same dinner.
• The two mothers-in-law got along surprisingly well.
• He credits his mothers-in-law, from two marriages, with teaching him to cook.
• Several mothers-in-law shared parenting advice at the baby shower.
• The sitcom's plot centered on rival mothers-in-law competing for attention.
• Both mothers-in-law offered to help plan the wedding.
• She keeps in touch with her mothers-in-law from both of her marriages.
• The support group connected mothers-in-law navigating blended families.
• Their mothers-in-law exchanged recipes during the holiday visit.
• The families joked about how differently their mothers-in-law approached grandparenting.
• Their mothers-in-law compared notes on the best local pediatricians.
• The family portrait included mothers-in-law standing on either side of the couple.
• Both mothers-in-law volunteered to host the holiday dinner this year.
• She thanked her mothers-in-law for their support during the move.
• Both mothers-in-law brought homemade desserts to the reunion.
• The retreat welcomed mothers-in-law seeking advice on blended families.
• Their mothers-in-law exchanged phone numbers to plan future visits.
• The novel follows two mothers-in-law forced to share a small apartment.
• Both mothers-in-law agreed to split the cost of the venue.