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POSSESSIVES

1. Nouns: Possessives of singular nouns, even if ending with an s or z sound, are usually formed with 's. If a triple sibilant in consecutive syllables or a difficult pronunciation results, an apostrophe alone may be added. Regularly formed plurals add apostrophe only; irregularly formed plurals add 's to the plural.
    Common nouns
 
                                Proper Nouns

    boy's, boys'
man's, men's
wife's, wives'
fox's, foxes'
waitress's, waitresses'
species'

  Mars's
Paris's
Mount St. Helens's
Dickens's
Athens's
Keats's, the Keatses'
Charles's, the Charleses'
Chris Johns's

Jesus'
Moses'
Texas'
United States'
Kansas'
Augustus'
New Orleans'
  A conventional exception applies to singular common nouns that end in an s or an s sound and are followed by a word that begins with s: appearance' sake, conscience' sake, goodness' sake.

2. Nouns with silent endings add 's:
    Malraux's
Arkansas's
Des Moines's

King Louis's
Peace Corps's
St. Tropez's
St. Croix's
Iroquois's
Rabelais's
Descartes's
3. Classical and hellenized names of more than one syllable that end with an unaccented syllable pronounced eez traditionally take only the apostrophe:
    Achilles'
Themistocles'
Euripides'

Archimedes'
Pericles'
Hercules'
Ramses'
Demosthenes'
Socrates'
Ulysses'
Xerxes'
4. Italicized terms add italic apostrophe and roman s:
    Yellow Rose's hull

the Winnie May's engine Time's article  
5. Compounds add the possessive ending to the final element:
    Lord and Taylor's
Alf the Magician's

John and Mary's house (joint possession)
But John's and Bob's houses (individual possession)
6. Other punctuation with possessive: The second comma setting off an apposition such as Jr. or D.C. may be dropped from the possessive. A quoted phrase cannot be made possessive; recast such a phrase.
    Joseph M. Blanton, Jr.'s memo

Washington, D.C.'s Anacostia River
7. Pronouns: Personal pronouns do not take the apostrophe: hers, its, theirs. Other pronouns have regular possessives: other's, others', but each other's, one another's.

8. Measures: Use an apostrophe in such expressions as an hour's work, eight hours' labor, a million dollars' worth, a month's wages.

9. False possessives: The apostrophe generally should not be used after a word that is more descriptive than possessive, except for a plural not ending in s: Explorers Hall, Diners Club, the Department of Veterans Affairs, teachers college but teacher's guide, St. Elizabeths Hospital, Teamsters Union, visitors center, children's hospital. But the Ladies' Home Journal, the National Governors' Association.

10. Double possessives: For clarity both of and the possessive form are often used when referring to something belonging to a person (or sometimes an animal):
             This dream of Nelson Mandela's (means Mandela has the dream)
             This dream of Nelson Mandela (means Mandela is the subject of the dream) 

Even when there is no ambiguity, the double possessive is idiomatic and correct:
            A friend of Nancie's
             A friend of mine

11. Singular noun with plural possessive: A singular noun can be used with a plural possessive for abstract qualities and figurative words:
            Four pilots crashed to their death, but four pilots ran to their cars
            the men earned their living; the spectators held their breath
            the depositors' curiosity was piqued; they kept their honor

12. Plural of a phrase containing a possessive: The plural is formed by adding s to the second word: master's degrees, debtor's prisons, ship's bells, lady's maids.