How to use an apostrophe

RULES FOR APOSTROPHE USE

Below are the key rules for using the apostrophe (').

These rules are those which apply in modern British English (as taught in High Schools and Universities throughout the Commonwealth).

PLURALS

When you add an 's' to make a plural, it NEVER takes an apostrophe. Never. There are no exceptions. The same is true for plurals of abbreviations - it is A.T.M.s, not A.T.M.'s and G.P.s not G.P.'s.

CONTRACTIONS

In contracted words - words formed by joining two words together and missing out some letters - the apostrophe takes the place of the missing letters so that:

Do not becomes Don't
I would becomes I'd
She will becomes She'll
We have becomes We've, and so on.

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Apostrophes are used to make a noun possessive - i.e. to show the ownership of an item.

With a singular noun, (such as John, horse, New Zealand), an apostrophe and an s are added at the end of the word to indicate ownership:- John's coat, the horse's tail, New Zealand's beautiful scenery. The exception to this is singular nouns which end in the letter s. For possessives, these are treated in the same way as plural nouns.

The possessive form of plural nouns and singular nouns ending in s (such as boys, dogs, soldiers, princess) is indicated by placing an apostrophe at the end of the word, but no s after the apostrophe:- the boys' boots, the dogs' collars, the soldiers' rations, the princess' gown.

The possessive form of a pronoun does not take an apostrophe at all but instead is an entirely separate word, as set out below:

Me - My/Mine
You - Your/Yours
He - His
She - Her/Hers
It - Its
We - Our/Ours
They - Their/Theirs
Who - Whose

Where alternatives are given, which you use will depend on the structure of the sentence, e.g.:

This is my/your/her/our/their house or

This house is mine/yours/hers/ours/theirs.

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COMMON MISTAKES

There are two very common mistakes in apostrophe use, and both come from not following the rules set out above:

Its, It's and Its': The possessive form of the word it is its, without an apostrophe anywhere, because it is a pronoun and follows the special rules for possessive pronouns. It's can only mean it is, or it has following the rules for contractions, and depending on context, and its' doesn't exist at all, since the plural of it is they.

Whose and who's: Once again, who is a pronoun, and following the rules for possessive pronouns, the correct possessive is whose. The word who's is a contraction of who is or who has.

If you use these rules you will never be using the apostrophe incorrectly, since although U.S. English is, as Gorgonzola points out below, less prescriptive, there is no situation where applying the rules above would be positively wrong, Strunk and White notwitstanding.

Before we begin, I must stress that you should never use an apostrophe to form an ordinary plural. Don't abuse the apostrophe.

This said, despite what you may hear from others, there are exceptions to the rule that plurals should not be formed with apostrophes. Specifically, you may use an apostrophe to form the plural of an abbreviation or a symbol. Thus,

"There were more Ph.D.'s at the reception than you could shake a stick at."

"Ice skater Valentina Semyovets received four 6.0's and six 5.9's for her astounding performance in the women's short program."

"Bullets over Biloxi is a riveting story of a gangster hideaway during the Roaring 20's."

"Mind your P's and Q's at the board meeting, do you hear me?"

are all perfectly good English usage, as is taught in high schools and colleges¹ across the United States.

Over the past 20 years or so, usage in the United States has begun to move towards omitting the apostrophe when forming the plurals of abbreviations and symbols. The trend is perfectly alright; the extra punctuation is not needed. However, to characterize its omission as 'correct' is a misrepresentation.

This difference does not result from 'ignorance'. It does not even result from 'confusion'. This is a cultural difference, and you would be well-advised to treat it as such.

Another cultural difference involves forming possessives from words which end in s. Some would assert that the 'correct' way to form such a possessive is to append an apostrophe but nothing else. This is decidedly not the case. The premier guide to English usage, The Elements of Style by William Strunk, Jr., and E. B. White, begins:²

I. Elementary Rules of Usage

1. Form the possessive singular of nouns by adding 's.

   Follow this rule whatever the consonant. Thus write,
   
   Charles's friend
   Burns's poems
   the witch's malice

¹For an example, see http://owl.english.purdue.edu/handouts/grammar/g_apost.html
²Strunk, William; White, E. B. The Elements of Style, Third Edition, © 1979 Macmillan Publishing Co, Inc., New York.