Apostrophes are used to show possession or omissions in words and phrases.
Possession
The rule is that the apostrophe always comes after the noun.
Example:
The University’s halls of residence
When something belongs to more than one person, then the apostrophe goes after the s.
Example:
The graduates’ certificates will be kept in departmental offices
Singular nouns ending in s are treated no differently and s should still be added:
Example:
The class’s teacher was absent from the room
Exceptions are proper nouns such as Jesus, Moses or Greek names ending in es.
Example:
Achilles’ heel was killing him
Some names ending in s can look and sound awkward when an extra s is applied. Sometimes it’s OK to leave out the additional s as long as your approach is consistent throughout the publication or document.
Example:
Dickens’ Great Expectations
Plurals of nouns omit the s after the apostrophe:
Example:
The classes’ timetables were confused
Where plural nouns that don’t end in s are used – children, women etc - the rule is the same. The apostrophe goes after the noun.
Example:
The women’s minibus runs until 11pm
Omission
Apostrophes are also used to show that letters are missed out of a word or phrase.
Example:
I’ll - I will
1995 - ’95
Where not to use apostrophes
Never use an apostrophe to form a plural with numbers and letters.
Examples:
1990s not 1990’s
Three As not three A’s
CDs not CD’s
Never use an apostrophe when it’s used in the possessive sense.
Example:
The University is conveniently located; its buildings are set on a beautiful riverside campus.