The possessive case of nouns is formed by adding 's or just an apostrophe to the "owner word."  For example, when we want to denote the legs belonging to the table, the house that John owns or inhabits, or the barking of the dogs, we write (respectively):  the table's legs, John's house, the dogs' barking.  (Note that, in the last, it was the barking of the dogs, not the barking of the dog.)  In these three examples, the owner words are:  table, John, dogs.

Think of the possessive case as a kind of shorthand.  Instead of going the long way and saying "a room for men," we can call it a men's room.  Instead of going the roundabout route and saying, "the boyfriend that Mary has" or "the boyfriend of Mary," we can say it with two words:  Mary's boyfriend.  Apostrophes are very useful, and it is a shame that many people don't know how to use them properly or omit them altogether.

RULES FOR FORMING THE POSSESSIVE

> If the owner word does not end in s, add 's.

> If the owner word ends in s and is plural (most but not all plural nouns end in s), add only an apostrophe.

> If the owner word ends in s and is singular, add 's – unless pronouncing the extra syllable is awkward.

(The last rule would apply to such words as boss or abyss and to many names and proper nouns, such as Charles, Thomas, Gladys, Mars, Venus.  However, we would probably want to add only an apostrophe to names such as Jesus, Moses, or Socrates because the extra syllable would be hard to say.)

Here are some examples:

Add 's to the owner word.
> the cover of the book = the book's cover
> the main point of the essay = the essay's main point
> the employees who work for a company = a company's employees
> toys belonging to (or for) children = children's toys
> the grades that the student got = the student's grades

Add only an apostrophe to the owner word.
> the coats that belong to the boys = the boys' coats
> the covers of several books = the books' covers
> the employees that work for these companies = these companies' employees
> the wedding anniversary of his parents = his parents' wedding anniversary
> the grades that the students got = the students' grades [compare: the grades that the student got]

Add 's to singular noun (when not awkward to say).
> the office belonging to my boss = my boss's office
> the wife of Prince Charles = Prince Charles's wife
> the orbit of Mars = Mars's orbit
> the house that Mr. Goss lives in = Mr. Goss's house

WARNINGS AND SPECIAL CASES

The possessive case is not used only in instances of literal ownership.  Sometimes it used in situations where "for" is implied.  Thus, clothing for women is women's clothing; books written for children are children's books; a room for ladies is a ladies' room, and a room for men is a men's room.

Literal ownership also does not apply to the possessive case as it is used with periods of time and sums of money.  A vacation of a week is a week's vacation (obviously, there is no such thing as "a weeks"), and a vacation of two weeks is two weeks' vacation.  If we have a year of experience, we have a year's experience, and if we have ten years of experience, we have ten years' experience.  We may buy one dollar's worth of gas (there is no such thing as one dollars) or five dollars' worth of gas.

Above all, we do NOT form the plural of nouns by adding 's.  Do NOT write, "I always get good grade's on my essay's, but my test's are not so great."  The nouns here are simple plurals (not possessives):  grades, essays, tests.  The exceptions are:
> Plural forms of lowercase letters and abbreviations with periods:  He always dots his i's with little circles and writes too many etc.'s.
> When needed to prevent confusion, use 's to show the plural of capital letters and of words referred to as words:  She gets straight A's.  He confuses his which's and his that's.
> When no confusion will result, we may add 's or just -s to constructions such as these:  the 1900's (or the 1900s); a row of 6's (or a row of 6s); a collection of CD's (or a collection of CDs).

Do NOT use the apostrophe to form the possessive case of personal pronouns.  It's is not the possessive of it but a contraction for it is, and there are no such words as your's, her's, our's, and their's.

For more information, see Chapter 19, pages 90-91.  Also see Study Guide No. 6, an online version of which appears here.

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