Singular possessive
The singular possessive is normally formed by adding ’s
the author’s work
the patient’s reaction
Smith’s paper
Newton’s laws
Names that end in ‑s, ‑z or ‑x
For names that end in ‑s, ‑z or ‑x, you can use an apostrophe either with ‑s or without ‑s.
In many cases both are possible:
Bayes’ theorem or Bayes’s theorem
Pepys’ diary or Pepys’s diary
Yeats’ poetry or Yeats’s poetry
However, use an apostrophe with ‑s if the possessive sounds unclear without ‑s:
Gauss’s law
Tallis’s music
Confucius’s analects
Linnaeus’s taxonomy
Erasmus’s work
Dylan Thomas’s Under Milk Wood
Lenz’s law
And use the apostrophe without ‑s if the possessive sounds or looks awkward with ‑s:
Archimedes’ principle
Achilles’ heel
Cervantes’ Don Quixote
Alternatively, you can paraphrase:
the reign of King James
the works of Tacitus
the Linnaean system
Marxist theory
Plural possessive
Most plural nouns simply add an apostrophe after the ‑s to form the possessive:
the authors’ work
patients’ reactions
participants’ feedback
Nouns whose plural does not end in -s
Plural nouns like men, women and children add ’s
men’s room
women’s rights
children’s hospital
Compound possessive
In compound possessives, add ’s to the last word of the compound:
the Security Council’s decision
William the Conqueror’s victory
Watson and Crick’s breakthrough
Possessive pronouns
The possessive pronouns do not have an apostrophe:
| his | ours | whose |
| hers | yours | |
| its | theirs |
But one’s does have an apostrophe:
One’s immediate reaction is …
Note: it’s with an apostrophe is not a possessive pronoun, it is short for it is. Similarly, who’s with an apostrophe is short for who is. Neither is used in formal writing.