Queen's English

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.

   With compound nouns it is a bit complex; the apostrophe "s" is unchanged at the end and the principal noun becomes plural: brother-in-laws' becomes brothers-in-laws'; passerby's becomes passersby's; postmaster general's becomes postmasters general's.

  In the case of joint possession (i.e., two people owning the same asset), only the last noun of the series is made possessive: "my brother and sister's home"; "John and Mary's car", "Robert, John, and Mary's partnership".  Note however, if the reference is to separate possessions, each noun has an apostrophe: "Jim and Sally's vacation plans" (if they are travelling together) but "Jim's and Sally's vacation plans" (if they are travelling separately); "Montreal's and Washington's subway systems are similar."; "My sister's and brother's weddings occurred within a week of each other".

   Finally, the plural pronoun: Add only the "s"; do not use apostrophes to make pronouns possessive.  Do not (unless you have no fear of losing your audience)!   It is such a common error.  Thus: "its climate"(referring to a city); "the house is hers"; "that sandwich is yours".  To add an apostrophe does not make it possessive; it is a verb contraction and changes the meaning completely: it's means it is; her's means her is (awkward and nonsensical); your's means your is (again, nonsense).  Here there is no room for discretion; it is wrong to insert the apostrophe (unless a verb contraction is intended as is it is).

   Ponder this: When a noun is as deeply engrained in our consciousness as McDonald's  (it comes only with the apostrophe built in) how do we make it possessive?  Do we approach silliness and say McDonald's's Big Mac?  Technically, we might, but practically, we would leave it alone: McDonald's Big Mac is just fine.

-- Ken Butera