The possessive case is used to indicate relationships between one person, place, or thing and another. However, it’s more accurate to call it by its alternate name, the genitive case (genitive means, essentially, “generation”), because in many uses, one person, place, or thing doesn’t actually belong to the other.
The genitive is indicated one of two ways: A singular noun is followed by an apostrophe and the letter s (as with book’s), and an apostrophe alone follows a plural noun that ends in s or es (as with teams’ or arches’). The genitive form of an irregular plural noun, one in which a change in spelling, rather than s or es, marks the word as having a plural form (such as men), is treated as if the word were a singular noun (men’s).
Here are examples of the seven categories of genitive use.
1. One type of genitive case is that denoting occupation, or ownership or possession, as in “She walked into Jane’s office” or “John’s car is being repaired.” (In all genitive forms, the noun to which the apostrophe or the apostrophe and the s are attached is called the dependent, or modifier, noun; the word with which it is associated is the head, or modified, noun.)
2. Another is relationship, as in “The school’s principal is retiring this year.” (This idea can also be represented by omitting the apostrophe and the s — “The school principal is retiring this year” — but the meaning is slightly different; in the latter sentence, the reference is to a person identified as the school principal, whereas the genitive form treats the school and the principal as separate entities.)
In this category, the idea can be expressed in a phrase beginning with the head noun: “The principal of the school is retiring this year.” The previous type is not so flexible; “She walked into the office of Jane” is awkward, and constructions such as “She walked into the office belonging to [or occupied by] Jane” are usually unnecessarily verbose.
3. The genitive can also be used to express agency, or representation, as in “The board’s secretary consulted the minutes from the last meeting.” (The secretary is a member of the board but technically doesn’t belong to it.) “The secretary of the board consulted the minutes from the last meeting” is also correct, but as in the previous example, the emphasis is slightly different, and the first version is more concise.
4. Description is another function of the genitive, as in “She admired the fabric’s glossy sheen.” (Loosely speaking, the glossy sheen “belongs” to the fabric, but the phrase is, strictly, speaking, a description.)
5. Another category is that relating to the role of the person, place, or thing that serves as the subject of a sentence, as in “The officer’s report was conclusive.” (“The report of the officer was conclusive” has the same slight distinction of meaning as similar constructions in previous examples.)
6. Then there is the role of the person, place, or thing identified as the object of a sentence, as in “The baby’s delivery was uneventful” (which can be rendered “The delivery of the baby was uneventful” with a slight difference in the meaning).
7. The most troublesome genitive form is that in which a phrase including of is truncated, as in “He gave two weeks’ notice” in place of “He gave notice of two weeks”: Many writers mistakenly treat “two weeks” as simply a modifier of notice (“He gave two weeks notice”) rather than correctly including the apostrophe to indicate the genitive case.