A compound noun contains two or more words which join together to make a single noun. Compound nouns can be words written together, words that are hyphenated, or separate words that go together by meaning.
Most compound nouns contain at least one noun. The other word or words may be an adjective, preposition, or verb. The second word is almost always the main word, with the first word modifying it or adding to its meaning.
Compound words, a large group of words to which compound nouns belong, are expressed in three ways.
Sometimes, the hyphen disappears as the word is more widely used, and it becomes a closed word.
Here are types of compound words and examples. The asterisks indicate types of compound words that may be compound nouns.
There are no hard and fast rules concerning plurals of compound words, especially since some hyphens are omitted after time. In hyphenated words, usually the “s” goes at the end of the main word, like daughters-in-law or mayors-elect. Sometimes it is at the end, like in go-betweens and higher-ups. In the open form, the “s” is added to the main word, like: bills of fare, assistant secretaries of state, and notaries public.
To make a compound word possessive, you usually add an apostrophe “s” at the end of the word, like: mother-in-law’s car or five-year-old’s birthday. If the compound word is plural, it can get a little strange with two “s” sounds close together, like: “fathers-in-law’s attire”. If you can, it would be better to reword the sentence so the plural compound word does not need to be possessive, like: “The attire of the fathers-in-law.”