Where to put apostrophes in possessive forms

     [This is a fast-access FAQ excerpt.]
PRONOUNS
The ONLY personal possessive pronoun with an apostrophe is "one's".
.---------------------------------------------------------------------.
| The words "his", "its", "whose", "their" do NOT contain apostrophes. |
|        Nor do words like "hers", "ours", "yours", "theirs".          |
|                   (Would you say "mi'ne"?)                           |
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The forms "it's", "they're", and "who's" are contractions for "it
is" (or "it has"), "they are", and "who is" (or "who has")
respectively.  They have nothing to do with possessive pronouns.
The apostrophe does occur in the possessive case of indefinite
pronouns ("anybody's", "someone's", and so on).
NOUNS
1. The standard rule:  Use 's for the singular possessive, and a
   bare apostrophe after the plural suffix -s or -es for the plural
   possessive.  For example:
                Singular         Plural
      Nominative              dog             dogs
      Possessive              dog's           dogs'
2. Nouns ending with an [s] or [z] sound (this includes words ending
   in "x", "ce", and similar examples):  The plural suffix is -es
   rather than -s (unless there's already an "e" at the end, as in
   the "-ce" words), but otherwise the rule is the same as above:
                Singular         Plural
      Nominative             class          classes
      Possessive             class's        classes'
   (The possessive plural is what is wanted in "the Joneses'".
   This is short for "the Joneses' house", which is not "the
   Jones's house".)
   There are, however, examples where the singular possessive suffix
   is a bare apostrophe:
         Singular         Plural
      Nominative            patience        patiences
      Possessive            patience'       patiences'
   (In most such examples, the plural is rarely used.)  For nouns in
   this category, many people would consider the 's suffix and the
   bare apostrophe to be acceptable alternatives.  The rules listed
   below may be taken as "most common practice", but they are
   not absolute.
   A. The 's suffix is preferred for one-syllable words (grass's) or
   where the final syllable has a primary or secondary stress
   (collapse's);
   B. The bare apostrophe is preferred:
      - for words ending in -nce (stance');
      - for many classical names (Aristophanes', Jesus', Moses');
      - where the juxtaposition of two or more [s] sounds would
    cause an awkwardness in pronunciation (thesis').
   C. Usage is divided in the situation where the final [s] or [z]
      sound falls in an unstressed syllable (octopus'/octopus's,
      phoenix's/phoenix', and so on).
   The question of which suffix is correct arises less often than
   one might imagine.  Instead of saying "the crisis' start" or "the
   crisis's start", most native speakers of English would say "the
   start of the crisis", thus avoiding the problem.
3. Plurals not ending in s:  Use 's for the possessive plural
   (men's, people's, sheep's).
HISTORY
   For those who want to know where the apostrophe came from, here
is how it probably happened.  Some of this is well documented, some
is guesswork on my part.
   Back in the days when English had many more inflections than it
now has, the most common suffix for the genitive singular was -es.
(There were several noun declensions, so that not all nouns fitted
this pattern; but this could be considered to be the "most regular"
case.)  For example: mann (=man), mannes (=of the man).  Over time
there developed a tendency to stop pronouncing the unstressed "e",
so that "mannes" became "mann's".  The apostrophe stands for the
omitted letter.
   (Modern German still has -es as the genitive suffix for many
nouns.  The Germans did not stop pronouncing their unstressed "e"s,
so the case suffix is still written as -es.)
   Pronouns were also inflected, but not in the same way.  (They
were all fairly irregular, as they still are today.)  The genitive
form of "hit" (=it) was "his" (=its).  As "his" evolved into "its",
there was no "e" to drop, therefore no logical reason to insert an
apostrophe.
   The "its" and "it's" forms did coexist in the 17th and early 18th
century, but today the "its" form is considered to be the only
correct spelling.
   Plural nouns are harder to explain.  The most common genitive
plural inflection was -a, which is quite unrelated to our modern
-s'.  My best guess is that most of the old plural suffixes were
replaced by -s under the influence of French; and that subsequently
the rules for forming singular possessives were extended to the
plurals.  If this is what happened, then a hypothetical -s's plural
possessive suffix would immediately collapse to -s', in the same way
as for many singular nouns ending in "s".  There was in any case a
long period where spelling was a lot less standardized than it is
today, so one should not think in terms of any sort of "standard
rule" existing during the transitional period.
NOTE FOR NON-ENGLISH-SPEAKERS
   The apostrophe in these cases normally has no effect on
pronunciation.  Thus dogs, dog's, and dogs' all sound the same.  The
exception is where the apostrophe separates two "s"s, and then it is
pronounced as an unstressed schwa.  Thus class's, classes, and
classes' are all pronounced as /klA:s@z/.
   For nouns where there is some difference of opinion over whether
the possessive suffix should be -'s or a bare apostrophe (that is,
those nouns where a final unstressed syllable ends with an [s] or
[z] sound) some native speakers use a lengthened final consonant
intermediate between /z/ and /z@z/.  This is, however, a fine and
almost inaudible distinction.
OTHER COMMENTS
   One occasionally hears that "John's dog" is an abbreviation for
"John his dog".  It is more likely that the derivation went in the
opposite direction, i.e.:
   Johnes hund => John's hound => Johnny's dog => John 'is dog
with the "John his dog" form coming into use only briefly before
disappearing from modern English.
   Using an apostrophe in a plural which is not a possessive form is
almost never recommended by prescriptivists.  The only situation
where it is recommended is where visual confusion would otherwise
result, as for example in the sentence "Mind your p's and q's".  In
forms like "the 1980s" or "two CPUs", apostrophes are not
recommended.
   It is correct to use an apostrophe to indicate missing letters,
in contractions like "aren't", "isn't", "it's" (= it is or it has).
Be careful in these cases to put the apostrophe in the correct
place.  The apostrophe replaces the missing letter(s); it does not
replace the space between words.