Like teenagers and salespeople, apostrophes are frequently there when they're not wanted, and not to be seen when they're needed. It seems that some people have noticed that apostrophes often go with the letter s ... so they put them in just in case. Because English uses an s to indicate a plural, this means they add an apostrophe whenever they have two of something: 'sausage's', 'cake's' or 'barbecue's'! But apostrophes are not used for plurals - they're used to indicate contractions and possessives.

Contraction is missing out a letter or letters - such as in 'it's' for 'it is', and 'isn't' for 'is not'.

The apostrophe always takes the position of the missing letter or letters. It doesn't go in the gap between the words; as, for example, in 'is'nt', an incorrect form that is often seen.

The possessive is used to show relationship, as in 'John's car' and 'the pupil's teacher'. These examples show how the possessive is usually expressed: by ''s'. If the word already ends in an s because it's plural - for example, 'pupils' - it needs only the apostrophe: "the pupils' teacher", not 'the pupils's teacher'. You wouldn't say 'pupils's' ('pupilziz'), so you don't write it either. If the word happens to end in an s anyway, it's optional to put an s after the apostrophe; for example, 'the class' teacher' or 'the class's teacher'. For some words, the rules are further refined, but these guidelines are sufficient for most.

The exceptions to the use of apostrophes in possessives are the possessive pronouns, such as 'hers', 'ours', 'theirs' and - the one that people so often get wrong - 'its'. When an 'it' owns something, it doesn't need an apostrophe: Virtue is its own reward. The trouble is, many of us are so used to seeing an apostrophe in 'it's' standing for 'it is' that we think the word can't be right without it. If you're unsure whether to punctuate 'its', ask yourself: "Would 'it is' make sense in its place?" If it wouldn't, leave out the apostrophe.