The apostrophe is the most commonly misused punctuation mark. There are two uses for apostrophes:
- contractions, and
- possessives
Contractions are words where one or more letter has been omitted: the apostrophe is used to indicate that letters have been removed. Some of the most common examples are:
- it’s (it is, it has)
- we’ll (we will, we shall)
- can’t (can not)
- won’t (will not)
Contractions can be used in formal writing, but you should use them only rarely as they make writing more chatty.
Some words feature an apostrophe because they were once contracted, but now we only ever use the contracted form:
- o’clock
- Hallowe’en
- ne’er-do-well
Possessives indicate that something belongs to someone. The place of the apostrophe depends on whether the possessor is singular or plural. A singular possessive goes before the possessive 's'; a plural possessive goes after the possessive 's'.
- the computer's code (one computer)
- the university's mission, (one university)
Some people find it useful to think of a singular possessive apostrophe as:
the computer (its) code
the university (its) mission
with the apostrophe showing that a word is missing (computer's code, university's mission) in the same way as it shows a letter is missing in a contraction (can't, won't).
- Professor Phillips's paper (one Professor Phillips - Professor Phillips (his) paper)
- the United States's demands (one United States - The United States (its) demands)
I have included Phillips and States because, when a noun ends in 's', it is often difficult to decide whether putting an apostrophe after the 's' and leaving out the possessive s, e.g. Phillips' or States', is correct or not. The rule is that if you would pronounce the s twice, i.e. Phillipses, you should put in the apostrophe. So, I would suggest Bill Gates's fortune (one Bill Gates) but Quintin Cutts' insights (one Quintin Cutts)
- the papers' merits (many papers - the papers (their) merits)
- the file sharers' guilt (many file sharers - the file-sharers (their) guilt)
- the networks' profits (many networks - the networks (their) profits)
Something to remember: children and people are plurals, but plurals that don't end in 's' should have a possessive apostrophe placed before the possessive 's'
- the children’s kidnapping (many children - the children (their) kidnappin)
- the people's discontent (many people)
N. B. the possessive 'its'
The possessive 'its' causes endless confusion with apostrophes. ‘Its’ is the possessive form of ‘it’:
- copyright in its most basic form
- compression technology was its saviour
- microchipping is in its infancy
Like 'his' and 'hers', when being used in the possessive sense ‘its’ never takes an apostrophe. It takes an apostrophe only when it's a contraction.
- It's a pity that robot can't sing in its bath. (It is a pity that robot cannot sing in its bath.)
Another common mistake is to put an apostrophe in plain plurals i.e. plurals that just indicate more than one. Plain plurals never take apostrophes:
- the plural of user is users
- the plural of computer is computers
- the plural of CD is CDs
- the plural of DVD is DVDs
The incorrect use of apostrophes irritates markers, mainly because it's quite easy to get them right.