After sibilants in English, 's
is always pronounced in possessives, just as -es
is pronounced in plurals, so it must always be written. The following plurals and possessives are pronounced exactly the same:buses, foxes, platypuses, leeches
(plural)bus's, fox's, platypus's, leech's
(singular possessive)buses', foxes', platypuses', leeches'
The same rule applies to all modern proper names, whether first names or last names (even though native speakers aren't always aware of the rule):We have three Charleses and two Maxes in our class.
Is this Rex's or Chris's book?
Let's invite the Edwardses and the Robertses to dinner.
Whose novels do you like better, Dickens's or García Márquez's?
Were the rest of the Keatses supportive of Keats's poetry?
Exceptions are made only for certain biblical or classical names, or for names that end in the sound /-iz/:Sophocles' plays
However, in more formal writing it's usually better to form possessives with 'of' anyway:the plays of Sophocles
The bill of the platypus contains special sensory receptors.
Whose novels do you prefer, those of Dickens or those of García Márquez?
Cf. Ich bin Thomas LehrerinLüppertz' oder Lüppertz'sGenitiv-s und Wortende ein "S"Mutter von James"St. James's Park"