Q: The president of the college where I teach asked me why we use the possessive in “doctor’s appointment” when the appointment is the patient’s. She also wondered why we don’t use it in “dentist appointment.” I thought I’d better check with you before answering her.
A: We use the term “possessive” today to describe relationships that involve more than possession.
Before we started calling this form (or case) the “possessive,” it was called the “genitive.” These days, only grammarians and other language types use the term “genitive,” but in some ways that old term was less confusing.
Genitives involve relationships much wider than simple possession or ownership. For example: measurement (“a week’s vacation”), affiliation (“Sylvia’s book club”), kinship (“Percy’s cousin”), description (“a bachelor’s degree”), and so on.
To these examples I would add “doctor’s appointment,” a phrase that describes the kind of appointment, not who owns it. Literal ownership is not involved.
Why don’t we say “dentist’s appointment”? Well, many of us do in fact say it, but the additional “s” often isn’t heard because the “st” at the end of the noun runs into it.
I hope this helps. As I say in my grammar book Woe Is I, for an acquisitive society, we have an awful lot of trouble with possessives.
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