Graham's Grammar

Helping English language learners

What is the plural of mother-in-law?

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  • A mother-in-law is the mother of someone’s wife or husband.
  • So a father-in-law, son-in-law or daughter-in-law is the father, son or daughter of someone’s wife or husband.
  • “Mother-in-law is an example of a “compound noun” – a noun that is formed by joining two or more words together. Other examples include:
    • Mayor of London
    • textbook
    • DVD player
    • CD ROM
    • Prime Minister
    • Iceland
    • shopkeeper
    • website
    • bathroom
    • blackberry
    • check-in counter
    • car park
    • greenhouse
    • midfield
    • passer-by
    • New York
    • Secretary of State
    • Doctor of Medicine
  • As you can see from this list there is no universal rule about hyphens, whether the words are joined together without a space, the use of capital letters. In fact, you might find the same compound noun in different forms. For example: car-park, carpark or car park.
  • The rule about plurals is that usually the “s” is added to the end. But this isn’t the case for “mother-in-law”, “father-in-law” and so on. The plurals are “mothers-in-law”, “fathers-in-law” etc. The explanation of this is usually that “there are two mothers, not two laws”. The same rule applies to such compound nouns as Secretary of State (Secretaries of State) and Doctor of Medicine (Doctors of Medicine).
  • The possessive form of most compound nouns is usually straight forward. For example: “the shopkeeper’s children” (one shopkeeper, two or more children) or “the shopkeepers’ children” (two or more shopkeepers, each with at least one child).
  • The possessive form of compound nouns such as “mother-in-law” are a bit more complicated. “Mother-in-law’s” is used when one mother-in-law possesses one or more item. When two or more mothers-in law possess things the correct form is “mothers-in-law’s”. But this looks strange and wrong so it’s best to avoid it. A better approach might be to use something like: “The jewellery belonging to the mothers-in-law”

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