An apostrophe is used in a possessive form, like Esther’s family or Janet’s cigarettes, and this is the use of the apostrophe which causes most of the trouble. The basic rule is simple enough: a possessive form is spelled with ‘s at the end.Hence:
England’s navy my brother’s girlfriend Wittgenstein’s last book children’s shoes women’s clothing the aircraft’s black box somebody’s umbrella a week’s work my money’s worth
This rule applies in most cases even with a name ending in s:
Thomas’s job the bus’s arrival James’s fiancée Steve Davis’s victory
There are three types of exception. First, a plural noun which already ends in s takes only a following apostrophe:
the girls’ excitement my parents’ wedding both players’ injuries the Klingons’ attack the ladies’ room two weeks’ work
This is reasonable. We don’t pronounce these words with two esses, and so we don’t write two esses: nobody says *the girls’s excitement. But note that plurals that don’t end in s take the ordinary form: see the cases of children and women above.
Second, a name ending in s takes only an apostrophe if the possessive form is not pronounced with an extra s. Hence:
Socrates’ philosophy Saint Saens’ music Ulysses’ companions Aristophanes’ plays
Same reason: we don’t say *Ulysses’s companions, and so we don’t write the extra s.
The final class of exceptions is pronouns. Note the following:
He lost his book. Which seats are ours? The bull lowered its head. Whose are these spectacles?
Note in particular the spelling of possessive its. This word never takes an apostrophe: *The bull lowered it’s head. This is wrong, wrong, wrong — but it is one of the commonest of all punctuation errors. I have even met teachers of English who get this wrong. The conventional spelling its is no doubt totally illogical, but it’s nonetheless conventional, and spelling the possessive as it’s will cause many readers to turn up their noses at you. The mistake is very conspicuous, but fortunately it’s also easy to fix — there’s only one word — so learn the standard spelling. (There is an English word spelled it’s, of course, and indeed I’ve just used it in the preceding sentence, but this is not a possessive: it’s the contracted form of it is or of it has. And there is no English word spelled *its’ — this is another common error for its.)
The same goes for possessive whose: this cannot be spelled as *who’s, though again there is a word who’s, a contraction of who is or of who has, as in Who’s your friend? or Who’s got a corkscrew? Note, however, that the indefinite pronoun one forms an ordinary possessive one’s, as in One must choose one’s words carefully.
There is a further point about writing possessives: when you add an apostrophe-s or an apostrophe alone to form a possessive, the thing that comes before the apostrophe must be a real English word, and it must also be the right English word. Thus, for example, something like *ladie’s shoes is impossible, because there is no such word as *ladie. Moreover, a department in a shoeshop could not be called *lady’s shoes, because what the shop is selling is shoes for ladies, and not *shoes for lady, which is meaningless. The correct form is ladies’ shoes. (Compare that lady’s shoes, which is fine.)
Finally, while we’re discussing clothing departments, observe that there is at least one irritating exception: though we write men’s clothing, as usual, we write menswear as a single word, with no apostrophe. By historical accident, this has come to be regarded as a single word in English. But just this one: we do not write *womenswear or *childrenswear. Sorry.
Other resources:
@Panj:
In this particular construction, the ten minutes takes on a possessive form.
It is a ten-minute walk to the river.
It is ten minutes’ walk to the river.
It’s all in a day’s work.
@Forero:
Note that the plural ending -s applies to one noun only. For example, the plural form of “secretary of state” is “secretaries of state”.
However, the “possessive” -‘s or -‘ governs an entire noun phrase, as if it were a postposition:
The Secretary of State’s car = The car of the Secretary of State.
I saw you-know-who’s son today = I saw the son of you-know-who today.
Ten minutes’ walk = A walk of ten minutes.
A phrase used as an attribute in front of a noun usually does not show number, even if the same phrase after the noun does:
A ten-minute walk = A ten-minute-long walk = A walk ten minutes long = A walk ten minutes in length.
Such a phrase requires hyphens if it contains an adjective followed by a noun. Both the hyphen(s) and the lack of plurality help to indicate the extent of the attributive phrase.
From: http://forum.wordreference.com/showthread.php?t=120182
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