If 's shows possession then wouldn't it be used in the following sentences?

I want to buy a car's door.
You have broken my computer's screen.

  • 4
    You could, but we usually don't. There really is no possession involved. Just use "car" as a modifier for door, and "computer" as a modifier for screen. So just "car door" and "computer screen". These are objects well known by themselves.
    – user3169
    Mar 14 '16 at 19:37
  • @user3169 is correct. These phrases are idiomatic, so we don't use the possessive unless there is a particular reason to.
    – Era
    Mar 14 '16 at 19:49
  • second sentence can also be expressed as You have broken the screen of my computer
    – MAKZ
    Mar 14 '16 at 22:16
  • Why not just flip the sentence around and say You have broken my computer screen? sounds much better Mar 15 '16 at 5:47
  • What do you mean there really is no possession involved? It is Terry's arm. Shows that he is the owner of his arm. But when it comes to car's door. It doesnt show that the car owns the door? Mar 15 '16 at 18:25

Although there is a degree of idiomatic preference with regards to these phrases and the (lack of) apostrophe-s, there is a more subtle bit of schema that helps to inform this choice.

Are we talking definite or indefinite?

The comment from @Era actually displays this. When we are talking about a particular tire on the specific car, thus referring to a definitive, well-defined object, Era uses the apostrophe-s construction:

My car's front left tire.

But when @Era talks about a not-individually-defined door (the car could have one, but most have at least two, if not four or more), now referring to an indefinite object, the apostrophe-s construction is dropped.

My car door.

Possession or Modification?

@user3169 brings up the idea that possession isn't necessarily involved with respect to car door or computer monitor. A door can exist without ever being part of a car, and a monitor can exist without being part of a computer. Thus these adjectives do not add possession because they don't clearly define the object as belonging to (or in other words, as being possessed by) a specific object.

This doesn't mean they can't. Defining the tire as being THE front left tire of the car (as opposed to any old tire) assigns possession of that specific object to another specific object (This is my car. There are many like it, but this one is mine.)

So, In Conclusion

Generally speaking: if the object is indefinite (just any old X), it doesn't use the apostrophe; if the object is definite (not just any X, my X), it uses the apostrophe. Exceptions abound (this is English, after all) but it's a good rule of thumb.

  • Would the same rule applies if I use it on the second sentence? Would it be My car's front left door.? Mar 19 '16 at 10:17
  • Absolutely. By describing the car as "your car" you've definitively identified the car in relation to you. As I said in aside in my answer, "This is my car. There are many like it, but this one is mine." Mar 21 '16 at 16:38

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