The apostrophe ( ’ or ' ) is a punctuation mark, used in the English language, as well as many others which make use of the Latin alphabet. Within English it serves two main functions, namely to mark the possessives of all nouns and pronouns, and to mark omissions and contractions.
Although the apostrophe is widely used within English, it is probably the least understood and most abused of all punctuation marks. There are, however, very clear and simple rules on its use, which if followed correctly, should result in flawless use of the apostrophe.
Use the apostrophe with contractions. The apostrophe always replaces the letter or letters which have been removed and is inserted at the point where the removed letters were. However, contractions should be avoided at all costs in formal writing.
do not = don't
you are = you're
they are = they're (NEVER their, or there)
he is = he's
The apostrophe can be used to indicate possession. For singular possession, the apostrophe is placed before the "s".
the ball of the dog = the dog's ball
the shoes belonging to Peter = Peter's shoes
the tie belonging to Mr. Smith = Mr. Smith's tie
However, when it comes to proper nouns that end in an "s" or a "z" sound, it is preferred, although not required, to have the second "s" added in possessive form.
the car belonging to Mr. Jones = Mr. Jones's car
the weather of Kansas = Kansas's weather
Use the apostrophe where the noun that should follow is implied. In this case, the apostrophe will not only indicate possession, but also the implied noun, which has been removed.
This was his father's, not his, book.
Used instead of saying "This was his father's book, not his book.
In cases of plural possession, the noun must always be made plural first. Then the apostrophe follows immediately after.
the cars belonging to two boys = two boys' cars
the awards belonging to two actresses = two actresses' awards
Once again, be careful when dealing with plural possession and proper nouns, especially those ending in an "s" or a "z" sound. The difference is shown below.
The house where Mr and Mrs Smith live = the Smiths' house,
The house where Mr and Mrs Jones live = the Joneses' house
Apostrophes are not to be used for the plural of a proper noun.
We visited the Smiths while on holiday. (Not Smith's or Smiths')
The Joneses own a cat and a dog. (Not Jones' or Joneses')
With a singular compound noun, possession is shown by adding 's at the end of the word.
the cat belonging to my mother-in-law = my mother-in-law's cat
When confronted with a plural compound noun, first form the plural and then use the apostrophe.
the golf clubs belonging to my two brothers-in-law = my brothers-in-law's golf clubs
When more than one person is being discussed, only use the apostrophe and s after the second name, if the people possess the same item.
Peter and Lucy's house is next door.
Peter's and Lucy's job applications were unsuccessful.
This is an extension of Rule 3 above. In this case, although both parties submitted job applications, they were not the same application, thus the double use of the apostrophe, rather than saying "Peter's application and Lucy's application". However, if they made a joint application, for example, for a bond, you could say "Peter and Lucy's bond application was successful."
Never use an apostrophe with the possessive pronouns: his, hers, its, theirs, ours, yours, whose, etc., as these already indicate possession and thus do not require an apostrophe.
This book is his, not hers.
The car is theirs.
The only time an apostrophe is used for "it's" is when it is a contraction for "it is" or "it has".
It is warm today = It's warm today
It has been a great pleasure talking to you = It's been a great pleasure talking to you
NEVER the dog chased it's tail.
The plurals for capital letters and numbers which are used as nouns are not formed with apostrophes.
She spoke to two Ph.Ds
She went to two Ph.D's offices. (In this case it is a plural possessive - the offices of the two Ph.Ds)
Alice learned her ABCs.
Dates are always written as the 2000s and not the 2000's. Likewise, the century can be dropped (and replaced with an apostrophe), but are still written as the '80s or the mid-'90s and not as the '90's or the mid-'70's
The exception to the rule is where the meaning would become unclear if apostrophes were not used.
Remember to cross your T's and dot your I's.
Without the apostrophe, "I's" could be read as "Is" and the sentence loses its (NOT it's!) meaning.
Use the possessive apostrophe in front of a verb gerund (a verb ending in "-ing").
Cindy's singing was very uplifting.
The exception to this, is if the gerund is preceded by a pronoun. In this case, the possessive pronoun (his, hers, theirs, etc.) is used with no apostrophe.
I appreciate your helping me with this problem.
His singing was very uplifting.
- Eats, Shoots and Leaves: The Zero Tolerance Approach to Punctuation; Truss, Lynne, 2003, Gotham Books