Plurals
Updated February 23, 2017 | Factmonster Staff 
The Question:
How do you make a plural out of mother-in-law?
The Answer:
Think about the noun you are using. Everything after the noun is modifying the noun, so what you want to do is simply make the noun a plural: mothers-in-law.
This goes for other compound nouns, too. The plural of heir apparent is heirs apparent; of attorney-general is attorneys-general; of man-of-war is men-of-war; of notary public is notaries public; and so on.
-The Fact Monster
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