Apostrophe of ownership or possession
Possession is when one noun has or owns another noun, shown with an apostrophe.
Example: The dog’s bowl is empty.
The apostrophe here is telling the reader that the bowl belongs to the dog.
An apostrophe is also used to tell the reader that a group has ownership of something.
Example: The dogs’ bowl is empty.
The apostrophe here is telling the reader that more than one dog owns the bowl.
When a person’s name ends in ‘s’ you can add an apostrophe and s to show possession.
Example: Jones’s theory revolutionised the discipline.
To show joint ownership, add an apostrophe and s to the last word only
Example: Green and Johnson’s research has been internationally recognised.
If ownership is not joint, add an apostrophe and s to each name.
Example: Green’s and Johnson’s research have been internationally recognised.
Apostrophe of contraction
To indicate contraction, that is, the shortening of a word by the omission of a letter.
Examples: Don’t is the contracted word for ‘do not’.
I’d is the contracted word for ‘I would’.
We do not use contracted words in formal academic writing.
A common mistake
It’s and its
In the word ‘it’s’ the apostrophe indicates a contraction of ‘it is’ or ‘it has’.
“Its” on the other hand, is a possessive personal pronoun, meaning ‘of it’ or ‘belonging to it’.
Whenever you write the word ‘it’s’, ask yourself if you mean to say ‘it is’. If not, then choose ‘its’.