For most singular nouns the possessive is found simply by adding an apostrophe and an “s”: cat’s dinner; student’s books; the Johnson’s apartment. Where a plural noun ends in “s” to make it possessive we add an apostrophe after the “s” and nothing more: cats’ dinner; students’ books, Johnsons’ apartment.
It is a bit more complicated when we consider words ending in a sibilant sound (words that end in “s”, “ce”, “x”, or “z”). Often to make these words plural, we add “es” with an apostrophe but no “s”: Jones’s (singular possessive) becomes Joneses’ (plural possessive); actress’s becomes actresses’; box’s becomes boxes’; quiz’s becomes quizzes’.
However there are situations where there can be too many “s” sounds, and you may consider dropping the “es” in which event the “‘s” would be dropped in favor of the apostrophe only: Dickens’s (singular) to Dickens’ (in lieu of Dickenes’); Williams to Williams’ (in lieu of Williamses’). You have the option here; either is acceptable.
There are words ending in the singular that require only an apostrophe to make them possessive when coupled with certain nouns: Achilles’ heel; Graves’ disease; Brahms’ lullaby, for goodness’ sake; for appearance’ sake. There is apparently no logic to support these uses – – they are idioms and have just developed that way. Posted in Queen’s English / Latin Lovers