When using an apostrophe to mark a possessive case, it is vital to know whether the person or object doing the possessing is singular or plural.

The rules

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The rules are simple:

  1. When the person or object doing the possessing is singular, you always add ' to denote that it is doing the possessing.
    • Examples:
      • A single man possessing a hat:  The man's hat.
      • The barking of a particular dog:  That dog's incessant barking.
      • The legs of a singular table:  My table's legs.
      • The waves of a single ocean:  The ocean's waves.
      • The labour performed within a given day:  All in a day's work.
      • Problems not possessed by anybody:  Nobody's problems.
      • Problems possessed by each other:  Each other's problems.
      • The evil plan of a single attorney-general:  The attorney-general's evil plan.
      • A list made by a single organisation:  The F. B. I.'s most-wanted list.
      • The heal of Achilles:  Achilles's heel
      • The rebellion led by Daniel Shays:  Shays's Rebellion
      • The staff of Moses:  Moses's staff
        • Pronounced:  moseses staff
      • The disciples of Jesus:  Jesus's disciples
        • Pronounced:  jesuses disciples
      • The brother of a single person:  Jacob Grimm's brother, Wilhelm.
        • Pronounced:  jacob grimms brother wilhelm
      • The books possessed or written by Charles Dickens:  Charles Dickens's books
        • Pronounced:  charles dickenses books
      • The philosophy of a single philosopher:  Socrates's philosophy.
        • Pronounced:  socrateses philosophy
      • The lack of relevancy of a single official:  The Lord Warden of the Cinque Ports's irrelevancy.
        • Pronounced:  the lord warden of the cinque portses irrelevancy
      • The McNuggets produced by a single restaurant named McDonald's:  The local McDonald's's McNuggets.
        • Pronounced:  the local micdonaldses micnuggets
      • The first track of a single album titled His 'n' HersHis 'n' Hers's first track.
        • Pronounced:  his en herses first track
  2. When the person or object doing the possessing is plural—a normal plural, where s or es is added to the singular to form it into a plural—you always add '  to denote that it is doing the possessing.
    • Examples:
      • The barking of multiple dog:  Those dogs' incessant barking.
      • The legs of a various table:  My tables' legs.
      • The waves of various ocean:  The oceans' waves.
      • Problems possessed by others:  The others' problems.
      • The single heal collectively possessed by multiple persons with the same name, Achille:  The Achilles' heel
        • Pronounced:  the achilles heal
      • The published compendium of multiple persons with the same surname, Grimm:  Grimms' Fairy Tales.
        • Pronounced:  grimms fairy tales
      • The books collectively possessed or collectively written by multiple persons with the same name, Charles Dicken:  Charles Dickens' books
        • Pronounced:  charles dickens books
      • The philosophy of multiple persons with the same name, SocrateThe Socrates' philosophy.
        • Pronounced:  The Socrates philosophy
      • The McNuggets produced by the various McDonald's branches:  McDonald's' McNuggets.
        • Pronounced:  micdonaldss micnuggets
  3. When the person or object doing the possessing is plural—where plurality is not denoted by adding s or es to the singular—you always add ' to denote that it is doing the possessing.
    • Examples:
      • Multiple men possessing a single hat:  The men's hat.
      • Multiple men possessing a multiple hats:  The men's hats.
      • Multiple children possessing a single game:  The children's game.
      • Multiple children possessing a multiple games:  The children's games.
      • A single evil plan held in common by multiple attorneys-general:  The attorneys-general's evil plan.
        • Pronounced:  the attorneys generals evil plan
      • Multiple evil plan held, whether individually or in common, by multiple attorneys-general:  The attorneys-general's evil plans.
        • Pronounced:  the attorneys generals evil plans
      • The legs of multiple people:  People's legs.
        • Pronounced:  peoples legs
      • The various sides of various dice:  The dice's various sides.
        • Pronounced:  the dices various sides
      • The lack of relevancy of multiple officials who have held the same title:  The Lords Warden of the Cinque Ports's irrelevancy.
        • Pronounced:  the lords warden of the cinque portses irrelevancy
  4. Exceptions:
    • Her
    • Hers
    • His
    • Its
      • It's was originally proper, was was dropped in the early 1800s.  It now refers exclusively to contractions of it is.
    • Mine
    • My
    • Their
    • Theirs
    • Our
    • Ours
    • Whose
    • Your
    • Yours

How placement disambiguates

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Each of these four phrases has a distinct meaning:

Phrase

Meaning
  • The libertarian outlook of a neighbour of a single friend of yours
  • Your friend's neighbours' libertarian outlook
  • The libertarian outlook of multiple neighbours of a single friend of yours
  • Your friends' neighbour's libertarian outlook
  • The libertarian outlook of a single neighbour of multiple friends of yours
  • Your friends' neighbours' libertarian outlook
  • The libertarian outlook of various neighbours of multiple friends of yours

Each of these three sentences has a distinct meaning:

Sentence

Explanation
  • The king said, "Those things are my subject's."
  • The king is claiming that those things belong to one of his subjects, or perhaps to his only subject.
  • The king said, "Those things are my subjects'."
  • The king is claiming that those things, whether individually or in common, belong to various subjects of his.
  • The king said, "Those things are my subjects."
  • The king is referring to his subjects as his "things."