Editorial Style Guide
General Style Preferences
If it will not lead to confusion, the plural of letters used as nouns may be formed by adding “s” alone: the ABCs. For plurals of single letters, use an apostrophe before the “s”: the three R’s, straight A’s. For plurals of figures, add an “s” alone: the 1970s, temperatures in the 30s.
Plural nouns ending in “s” take only the apostrophe: the cows’ feed, the boys’ clothes, states’ rights. Singular common nouns ending in “s” take an apostrophe before the ending “s” (the witness’s testimony); however, use only the apostrophe when the following word begins with an “s” (the witness’ story).
Singular proper names ending in “s” take only an apostrophe: Achilles’ heel, Dickens’ novels, Tennessee Williams’ plays.