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.
- politics' true meaning
- economics' forerunners
- this species' first record (or, better, the first record of this species)
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.
- the United States' role in international law
- Highland Hills' late mayor
- Calloway Gardens' former curator
- the National Academy of Sciences' new policy
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.
- Euripides' tragedies
- the Ganges' source
- Xerxes' armies
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.
- Descartes' three dreams
- the marquis' mother
- Francois' efforts to learn English
- Vaucouleurs' assistance to Joan of Arc
- Albert Camus' novels (the s in Albert Camus is unpronounced)but
- Raoul Camus's anthology (the s in Raoul Camus is pronounced)
- W.E.B. DuBois's writings (the s in DuBois is pronounced)
Other exceptions. For ... sake expressions traditionally omit the s when the noun ends in an s or an s sound.
- for righteousness' sake
- for goodness' sake
- for Jesus' sake but
- Jesus's contemporaries
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.
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