Compound nouns are nouns that are formed by the combination of two
or more nouns or nouns with a preposition,
adjective or adverb.
For
example: armchair, bookcase, brother-in-law, toothpaste, bedroom
There
are a couple of rules to follow when making compound nouns plural.
Rules for forming plurals
from compound nouns:
A.
Some plural compounds can be formed by adding “-s” to the main element
(word) in the compound noun. For example:
son-in-law → sons-in-law.
B.
Some plural compounds can be formed by adding “-s” to the last element.
For example:
grown-up → grown-ups.