Here are some examples of when the letter s is added to a possessive form.
Nouns plural in form, singular in meaning. When the singular form of a noun ending in s looks like a plural and the plural form is the same as the singular, the possessive of both singular and plural is formed by the addition of an apostrophe only. If ambiguity threatens, use of to avoid the possessive.
The same rule applies when the name of a place or an organization (or the last element in a name) is a plural form ending in s, such as the United States, even though the entity is singular.
Names like "Euripides." The possessive is formed without an additional s for a name of two or more syllables that ends in an eez sound.
Words and names ending in unpronounced "s." To avoid an awkward appearance, an apostrophe without an s may be used for the possessive of singular words and names ending in an unpronounced s. Opt for this practice only if you are comfortable with it and are certain that the s is indeed unpronounced.
Other exceptions. For ... sake expressions traditionally omit the s when the noun ends in an s or an s sound.
Where neither an s nor an apostrophe alone looks right (as with such names as Isis), avoid the possessive and use of instead.
Source: The Chicago Manual of Style.
Get a copy of our newest e-book: Why Writers Need Professional Proofreaders. See some common mistakes that get by spell and grammar checkers and writers. Click here to get your copy. |