The Editor’s Manual
Free learning resource on English grammar, punctuation, usage, and style.
Avoid using an apostrophe before the “s” in a plural, unless not using one would result in confusion (e.g., two “i’s” in “iridescent”).
Possessive forms of nouns (with an apostrophe either before or after an “s”) and pronouns (without an apostrophe) indicate a relationship of belonging.
Use an apostrophe to form a possessive noun (“the cat’s tail”) or a contraction (“don’t”). Only rarely is an apostrophe used to form a plural (“p’s and q’s”).
Form the plural of an acronym or other abbreviation simply by adding “s” to the end (URLs, CEOs). No apostrophe is necessary.