I was thinking of my brother’s wife and my husband’s sister today. Each one separately is my sister-in-law, right? But, what do I call them when I want to refer to the two of them together?
Sister-in-law is a compound noun. A compound noun is just a fancy way of saying you’re using two or more words joined together to describe a person, place or thing. Many compound nouns are written as one word — for example, bathroom, doorjamb, and baseball. Others, such as bottle opener, dog food and car park, are written as two words. Still others are written with hyphens, which brings us back to sister-in-law.
The trick for making a hyphenated compound noun plural is to determine the main noun and then add an s to it. What’s the main noun in sister-in-law? Sister. Law simply tells you what kind of sister she is. So, you can invite your two sisters-in-law to a party, or you could have a meeting with two editors-in-chief. If you tied for second place in a writing contest, you’d be one of the runners-up.
My rule for compound words is to always look them up. A good dictionary will tell you the proper form for a compound noun and its plural.
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