Within the realm of punctuation, apostrophes on plurals and possessives can also lead to questions. There are patterns for forming plurals, though, and so that you do not have to wonder about when and where to put an apostrophe.
When the possessor is a regular singular noun
The most basic case of possession is signifying when a single person or object has ownership over something else. The rule for this in English is simply to add an apostrophe + "s" to the end of the noun that is doing the possessing. For example, if Alison owns a blue car, then it is Alison's car. Easy enough.
When the possessor is a singular noun that ends in "s"
Some names and objects, even when singular, end in "s". For instance, what if Jess has two pencils? Jess also has a name that ends in "s", which means that putting an apostrophe and "s" after it might look funny, even though we know that is the standard way to denote possession in English. There are two ways to write the pencils of Jess:
1. An apostrophe and "s" after her name, just like always, even though it looks a little clunky:
Jess's pencils are sharp.
2. For those who do not like the look of so much "s", you can eliminate the "s" after the apostrophe to make a cleaner possessive:
Jess' pencils are sharp.
Both of the above ways are grammatically acceptable. Personally, I prefer the second way because it sort of seems like the path of least resistance, in terms of punctuation marks. However, some may prefer the first way because it is more clearly obvious that Jess is a possessive in this context.
The same applies to a family name that ends in "s", such as the Jones family.
The Jones's dog barks a lot.
The Jones' dog barks a lot.
If a family name does not end in "s" then you must first make it plural and then add a possessive apostrophe if you are trying to convey that the whole family is the possessor.
The two cats that belong to the Smith family are the Smiths' cats.
When the possessor is a plural noun
In general, the rule for making a singular noun a plural in English is to add an "s". When this is the case and the plural noun collectively possesses something, put the apostrophe after the "s" to show that the thing is owned by all of the members of the plural group and not just one.
If one kid has a popsicle, it's the kid's popsicle.
When a lot of kids have popsicles, they are the kids' popsicles. (And if multiple kids are sharing just one popsicle, that popsicle is still the kids' popsicle because all of the kids are in possession).
When the possessor is a plural noun that does not end in "s"
Children is the plural noun of child. So how do you say that the classroom belongs to all of the children? The same way you would with a singular noun: add an apostrophe + "s" to the end of the noun.
The children's classroom is painted yellow and blue.
Many people mistake this type of situation by writing childrens' because it resembles the situation described above in which an already plural group is possessing something (as in the kids' classroom). The difference here is that children is a collective noun, meaning it signifies a plural but acts as a singular noun would in the sentence. It is an irregular plural because it is not formed by simply adding an "s" to the end of its singular form (childs is not the plural of child, whereas kids is the plural of kid).
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