To determine whether you need to add -s, follow these steps:

1.  Write down the possessor or possessors.  Examples:

Sarah

the girls

women

Jesus

Arkansas


2.  Put an apostrophe after the possessor or possessors. Examples: 

Sarah'

the girls'

women'

Jesus'

Arkansas'


3.  Add the noun that is being possessed after the word, and pronounce the two words aloud to find out whether you add an additional "s" sound to the possessor when you say it. If you do, add an "s" after the apostrophe.

Examples:

Sarah'__sweater

Sarah's sweater

the girls'__ locker room

the girls' locker room

women'__ health

women's health

Jesus'__ sandals

Jesus's sandals or Jesus' sandals

Arkansas'__ rivers

Arkansas's rivers

 

As you can see in these examples, the use of an apostrophe is pronunciation driven and thus individualized, not determined by hard and fast rules in all cases.

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