The MHRA Style Guide Online
A Handbook for Authors and Editors • Third Edition
From Chapter 2, 'Spelling and Usage'
The possessive of nouns and indefinite pronouns is regularly formed by the addition of s preceded by the apostrophe:
the court’s decision, the doctor’s fee, the witness’s testimony, Smith’s elixir, no one’s fault, the children’s day out
The possessive forms of personal pronouns hers, its, theirs, yours do not have an apostrophe.
In plural nouns ending in -s the possessive is represented by the apostrophe alone:
the courts’ decisions, the doctors’ fees, the witnesses’ testimonies, MPs’ assistants
The possessive of proper names ending in a pronounced -s or -z is formed in the normal way by adding an apostrophe and s:
Alvarez’s criticism, Berlioz’s symphonies, Cervantes’s works, Dickens’s characters, in Inigo Jones’s day, Keats’s poems, Dylan Thomas’s use of language
French names ending in an unpronounced -s, -x, or -z also follow the normal rule and take an apostrophe and s:
Rabelais’s comedy, Descartes’s works, Malraux’s style, Cherbuliez’s novels
The possessive of names ending in -us also conforms to the normal rule:
Claudius’s successor, Herodotus’s Histories, Jesus’s parables, an empire greater than Darius’s
However, the possessive of Moses and of Greek names ending in -es (particularly those having more than two syllables) is usually formed by means of the apostrophe alone:
under Moses’ leadership, Demosthenes’ speeches, Sophocles’ plays, Xerxes’ campaigns