What's Mine Is Yours: Possession
Before using an apostrophe to show possession, first make sure the phrase you're questioning actually denotes possession and isn't simply a plural. For instance, in the phrase the babies' rattles, the babies possess rattles (so an apostrophe indicates this to readers); however, in the phrase the babies in their cribs, the babies aren't possessing anything and an apostrophe isn't needed.
Here are some guidelines to help you make sense of it all.
Guideline #1. If a singular noun doesn't end in -s, its possessive ends in -'s. Say what? Take a look at this sentence:
The cars engine was still running.
The word cars needs an apostrophe to indicate possession, but where does the apostrophe go?
Use this mental trick: Take the word that needs the apostrophe (cars) and the word that it's talking about (engine) and mentally turn the two words around so that the word you're wondering about is the object of a preposition. (This rule may be easier for you to understand this way: Turn the words around so that they form a phrase. Usually the phrase will use of, from, or belonging to.)
When you change cars engine around, you come up with engine of the car. Now look at the word car. Car is singular and doesn't end in -s, so the original should be punctuated -'s.. You should have:
The car's engine was still running.
Try the trick again with this sentence:
Donna Moores wallet was lying on the seat.
Mentally turn Donna Moores wallet around so that you have the wallet of (belonging to) Donna Moore.
After you've turned it around, you have the words Donna Moore, which is singular (in spite of being two words) and doesn't end in -s. That lets you know that you need to use -'s. The sentence should be punctuated this way:
Donna Moore's wallet was lying on the seat.
One of the most common mistakes with apostrophes comes with possessive pronouns (its, yours, his, hers, theirs, ours, whose). Remember that the only one of these words that ever takes an apostrophe is its, and that happens only when the word means it is.
Guideline #2. When you have plural nouns that end in -s (and most do), add an apostrophe after the final -s. This tells readers that you're talking about several people, places, or things. The same mental trick of turning the two words into a phrase applies.
This sentence talks about two girls who had been reported missing:
The girls coats were found at the bus station.
Now just apply the trick. Take the phrase girls coats, and turn it around so that you have coats of (belonging to) the girls.
When you've turned the phrase around this time, the word girls ends in -s. This lets you know that you should add an apostrophe after the -s in girls, so the sentence is punctuated this way:
The girls' coats were found at the bus station.
Although most English plurals end in -s or -es, our language has a number of exceptions (and didn't you know there would be?), such as children, women, and deer. If a plural doesn't end in -s, the possessive is formed with an -'s (that is, treat it as if it were singular).
Again, the turnaround trick applies. Take the sentence:
The childrens coats were covered with mud.
Mentally turn childrens coats into the phrase coats of the children. Since children doesn't end in -s, its possessive would be -'s; so the correct punctuation would be:
The children's coats were covered with mud.
So far, so good? You have just one tricky part left to consider. It concerns singular words that end in -s. Two ways of punctuating these words are common. Guideline #3 is used more often than Guideline #4, but many people find that Guideline #4 is easier to grasp. You'll have to ask instructors or employers if they have a preference as to which you should follow.
Guideline #3. If a singular word ends in -s, form its possessive by adding -'s (except in situations in which pronunciation would be difficult, such as Moses or Achilles). Look at this sentence:
Julie Jones information was invaluable in locating the missing girls.
Applying the turnaround trick would make the phrase that needs the apostrophe read this way: information from Julie Jones.
Guideline #3 would tell you that, since Jones is singular and ends in -s, you'd form the possessive by adding -'s. Therefore, the sentence would be punctuated this way:
Julie Jones's information was invaluable in locating the missing girls.
However, you may be told to use another rule:
Guideline #4. If a singular word ends in -s, form its possessive by adding an apostrophe after the -s. In this case, the sentence would be written this way:
Julie Jones' information was invaluable in locating the missing girls.
If using Guideline #4 is okay with your teacher or employer, then you have to remember only two rules about placing the apostrophe in possessives:
After you mentally turn the phrase around, if the word in question doesn't end in -s, add -'s.
After you mentally turn the phrase around, if the word in question ends in -s, add an apostrophe after the -s.