Many times i get confused whether i should use 's with a noun or not. For example:

  • Engine parts or Engine's parts
  • Human brain or Human's brain
  • Wordpress plugin or Wordpress' plugin
  • Computer program or Computer's program etc.

I want to know why and when we use one or the other. What do you call such a group of words? Is it a phrase?
It should be noted that English is not my native language so pardon my ignorance.

  • It's context-dependent. E.g. "The engine/engine's parts are lying there" both work, whereas "Engine parts are getting very expensive" doesn't work for the "engine's" version - that would need a determiner, like "This engine's parts are getting very expensive", and it changes the sense of the sentence from engine parts in general to the parts of one specific engine.
    – Lawrence
    Jun 22, 2017 at 8:49
  • 3
  • @Lawrence wish you had posted your comment as an answer. Your comment is much helpful.
    – Shuji
    Jun 22, 2017 at 11:17
  • @ShujaatAliKhan Glad it helped. :)
    – Lawrence
    Jun 22, 2017 at 11:26
  • @ShujaatAliKhan I've converted my comment into an answer for you.
    – Lawrence
    Jun 22, 2017 at 22:53

2 Answers 2


The 's signifies possession. You add it when you want to stress that the second noun is the property of the first one. Without the 's, it's a more generic version.

For example, if you bring a friend to your garage, and he sees a bunch of bolts and screws lying on a workbench and asks "What're those?" then you can say "They are engine parts", and you would mean that they are things that go into engines, but they don't belong to any engine specifically, or it doesn't matter which engine they belong to. They might just be some parts you have for a later repair.

On the other hand, you could also say "Those are the engine's parts", which would indicate that they came from a specific engine and they still belong to it. Probably because you're repairing an engine and you removed them from it for cleaning, and they will be put back in later.

So whether to use the 's or not is usually part of the context; do you want to say that "the parts" belong to "the engine" or do you want to say that "the parts" are for "an engine"?

(same with the others, although they are less likely to come up.)

  • This is the rule of thumb. Admittedly, a pretty good one. There are various grey areas (writers' guild // writers guild; Achilles['] heel // Achilles tendon; nine days['] wonder; working mens club ...). The grey areas have already been discussed here at length, and the 'when to use the attributive rather than the Saxon genitive' question has also been asked and answered. Jun 22, 2017 at 9:30
  • @Erik Thanks for the detailed and clear explanation.
    – Shuji
    Jun 22, 2017 at 11:22

It's context-dependent.

Consider the following:

  1. The engine parts are lying there.
  2. The engine's parts are lying there.
  3. Engine parts are getting very expensive.

The first two both work, while the "engine's" version of #3 requires a determiner, producing something like "This engine's parts are getting very expensive." However, doing this changes the sense of the sentence from engine parts in general to the parts of one specific engine.

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