The possessive of most singular nouns is formed by adding an apostrophe and an “s.” The possessive of plural nouns (except for a few irregular plurals that do not end in “s”) is formed by adding an apostrophe only. Examples: the horse’s mouth, a bass’s stripes, puppies’ paws, children’s literature.
This general rule covers most proper nouns, including names ending in “s,” “x,” or “z,” and both are singular and plural forms, as well as letters and numbers. Examples: Chicago’s lakefront, Massachusetts’s legislature, Burns’s poems, Marx’s theories, Berlioz’s works, the Lincolns’ marriage, FDR’s legacy, 2003’s heaviest storm.
When the singular form of the noun ending in “s” looks like the plural, and the plural form is the same as the singular, the possessive of both singular and plural is formed by adding only an apostrophe. Examples: politics’ true meaning, economics’ forerunners, this species’ earliest record.
The same rule applies when the name of a place or an organization is the plural form ending in “s,” such as the United States, even though the entity is singular. Examples: the United States’ role in world peace, Marvin Gardens’ former curator, Greenwood Hills’ last mayor.