Apostrophes are probably the most mis-used of punctuation marks. People leave them out when they should be there. They put them in when they shouldn't. Usually it doesn't matter, but sometimes the meaning of the sentence is unclear, which for the Punctuation Police is a punishable offence. English should be clear, because that's what it's there for.

The apostrophe has two main uses:

• Showing that something belongs to something.
• Showing that something has been left out.

These are probably equally common.

Showing that something belongs to something

• If a man owns a hat, then it is the man's hat. You add an s to man, but to show that possession is involved, you put an apostrophe between man and the s.

• It two men own a house, then it is the men's house. The rule is the same. Add apostrophe s.

• If the house belongs to two boys, the plural word already ends in s - you don't add another s, but you do add an apostrophe to show possesion: the boys' house. Think about the way you actually say it - you say boyz, not boyzuz, so there is no need for another s.

• The final case is when the word is singular and already ends in an s. An example would be Charles. When Charles owns a house, would you say "charlz house" or "charlzuz house"? Some people prefer one and some the other. If you go for the first one, then you would write it Charles' house with an apostrophe but no s. If you say "charlzuz", then you should write it as Charles's with an apostrophe and an s. Both of these are correct - it's a matter of taste which you prefer. You can see both in the titles of books: Bridget Jones's Diary has apostrophe s while ...

One final oddity is the word "its" meaning "belonging to it". You might think that it should have an apostrophe, but it doesn't. It is an exception to the general rule, and one you should memorise.

Showing Something is Left Out

The second main use of the apostrophe is to show that something has been left out. Most people, instead of saying "do not" will say "don't". The second o is left out, so we put in an apostrophe to show this. So we have:

don't means do not

I've means I have

she'll means she will

he's means he is or he has

it's means it is or it has

Some of these abbreviations are rather odd:

won't means will not. The apostrophe indicates the missing o in not, but what's that other o doing there?

shan't means shall not, but there should really be a second apostrophe to show the missing l. There isn't.

To re-iterate: it's means it is or it has. The apostrophe shows that something has been left out. It does not mean "belonging to it" - that would be written its.

Obscure Plurals

The third use of the apostrophe is much rarer, and as a result many people don't know about it. It is used for forming the plurals of some very short words, ones which are only one or two letters long.

If you have a letter P and another letter P, what have you got? How do you write the plural of the P? Opinions differ on this. One method is to use an apostrophe and an s, so you have P's. Some people don't like this because it is not anything belonging to the P, and nothing has been left out. They prefer to write Ps. That looks a bit odd, and the method doesn't really work for all letters: if I have two As, threes Is and some Ss, do you know what I'm talking about?

So A's, B's and C's are a valid way of talking about the plurals of letters.

The word "do" is a verb, but can also be used as a noun: it means a party or get together. "We're having a do tonight and you are invited." These events usually come singly, so we rarely need to talk about more than one of them. If there is more than one, we have a problem. The plural of "do" is pronounced "dooz", but how do we spell it? One acceptable way is do's. Many people don't like this spelling and think it is a mistake, but it has been used for centuries and was recognised as good grammar in the past. The alternative of "dos" looks like ir is pronounced "doss" so is not really very good.

Other short words that need an apostrophe for the plural are the singing terms which you probably know from the song "Do a deer". Do, re, mi, fa, so, la and ti are used for naming singing notes in the system known as 'solfa'. If you want to make the words plural, for example, if a song has three ti's in a row, you'll need the apostrophe s combination.

Glottal Stops

There is a fourth use of the apostrophe; it's not part of English, but you will sometimes see it in foreign names - the apostrophe represents the 'glottal stop' sound of Arabic languages which is pretty rare in English and doesn't have a special symbol. For example 'ayin, one of the letters of the Arabic alphabet starts with a glottal stop.