Overview of apostrophes

The Rules on When to Use Apostrophes

The rules on using apostrophes can be summarised:

• To replace missing letters
aren't
isn't
can't


• To show possession
Sarah's jumper
The dog's toy
The dogs' kennel


• In temporal (or time) expressions
3 months' wages
A day's leave
2 days' notice


• To show the plural of an awkward abbreviation, letter or number
BUY THREE CD'S.
Mind your p's and q's.
Your 5's look like 6's.

Unless it is covered by one of the categories above, an apostrophe should not be used.

The Most Common Mistake with Apostrophes

Lots of words in English end in the letter s. This happens for one of three reasons:

• It's a plural (e.g., cats, houses, dreams).
• It's a verb ending (e.g., he flies, she bakes, it likes).
• It's a singular noun that ends in s (e.g., Wales, crisis, humous).

You will notice that not one of the examples in the three bullets above has an apostrophe in it. This is correct. Do not add an apostrophe to a word just because it ends in s. That is a serious mistake.

Examples:

• Anteater's prefer termite's to ant's. 
• Anteaters prefer termites to ants. 
• Pearl's melt in vinegar. 
• Pearls melt in vinegar. 
• She flie's to New York tomorrow. 
• She flies to New York tomorrow. 
• Will you see Charle's after this meeting? 
• Will you see Charles after this meeting? 

The plurals of words ending in vowels (e.g., video, patio) are extremely prone to this error.

• Hire two video's for the price of three. 
• Hire two videos for the price of three. 

Putting an apostrophe before the s in the plural word is a very common mistake and is considered a grammatical howler.