Apostrophe rules for possessives are confusing because common practices vary. In fact, grammar and style guides reflect some differences of opinion on the question of possessive forms. Simple guidelines on apostrophes in possessives appear below, though if your company or department has its own established preferences for possessive formation, you should probably follow those practices.
3.8.1 Possessives of Singular Nouns
To form the possessive of a singular noun, add ’s.
a dog’s life
the boss’s daughter
Mr. Jones’s firm
Now, many journalists do not add an s when they form the possessive of a singular proper noun already ending in s. Therefore, don’t be surprised if what you see in the news sometimes differs from the guideline illustrated above. If you prefer to follow that fairly common journalistic style, you will have plenty of company, but keep in mind that most non-journalist language experts — as well as leading periodicals such as the New York Times and the Washington Post — actually prefer Mr. Jones’s firm to Mr. Jones’ firm.
Regardless of your position on what to do with Mr. Jones, there are some Biblical and classical names whose possessives are usually s-free:
3.8.2 Possessives of Plural Nouns
To form the possessive of a plural noun, generally add an apostrophe to the end of the word, as in:
the executives’ analysis
the Smiths’ house
Some plural nouns, however, do not end in s. In such cases, form a possessive by adding an ’s to the end of the word, as in:
the women’s room
the children’s school
the men’s sauna
The following forms are always mistakes:
3.8.3 Joint Ownership
Sometimes a possessive involves two or more nouns. Where there is joint ownership, the formation of the possessive depends on the meaning of the sentence. Compare these two examples:
Jill and Jack’s company
Jill’s and Jack’s desks
In the first example above, Jill and Jack share ownership of a company; in cases such as this, only the second possessive noun — here, Jack’s — shows the possessive form.
In the second example, Jill and Jack each have a desk. In this scenario, where ownership is not shared, both of the nouns reflect the possessive form — hence, Jill’s and Jack’s. If they shared a single large desk, however, the phrase would be Jill and Jack’s desk.
3.8.4 Possessive Pronouns
People often mistakenly add an apostrophe to possessive pronouns such as the following:
None of the possessives above should ever appear with an apostrophe. It’s is a word, but it is not a possessive pronoun; rather, it is a contraction for it is or it has.
If you are unsure whether to use its or it’s in a particular context, try substituting the words it is or it has. If the substitution doesn’t make sense, use the (apostrophe-free) possessive pronoun its.