Why is the clause "who was wearing a blue hat" in the first sentence(1) a defining one? It seems to me to me that this sentence is clear in its meaning if we omit this information and regard this sentence as a sentence containing a non-defining relative clause and separate "who was wearing a blue hat" with commas(2).

(1)A woman who was wearing a blue hat opened the door.

(2)A woman,who was wearing a blue hat,opened the door.

Thanks in advance.

  • For your exact text there wouldn't usually be much difference. But in principle #1 could be used if there were several women potentially "in scope" who might have opened the door - but speaker has specifically excluded them, and identified the one in a blue hat as the one who did it. Whereas in principle #2 could be used if there was only one woman "potentially in scope" - and speaker is just telling us a bit more about her. Commented Dec 14, 2022 at 17:39

2 Answers 2


The kind of commas you are talking about are parenthetical commas. They do the same job as parentheses, commonly known as brackets (like the ones I'm using now).

For example:

  • A woman, who was wearing a blue hat, opened the door.
  • A woman (who was wearing a blue hat) opened the door.

Parenthetical information is by definition non-defining. It is useful, but not essential, information. You should be able to skip the text within parentheses and the remainder of the sentence will still be grammatically correct.

You asked why the detail of a blue hat in your example text was a defining clause. Well, that's up to the writer to decide. If they think it is non-essential, the writer may make a detail parenthetical. If by 'why is it defining' you mean why is it essential information, that is something only the context could answer. Was the first person looking for someone in a blue hat? Did the blue hat identify the woman as being someone in particular?


The two sentences have two different subjects. In the first sentence, the commas indicate the end of the noun phrase that forms the subject.

In the first sentence, we know for sure that the blue hat is unimportant to understanding the intended meaning.

But in the second sentence, it's likely that the blue hat has some significance in the context. For instance, the blue hat might indicate a flight attendant, a nurse, or the queen opened the door. It could also help identify the person to the police if they've committed a crime. In those contexts, the blue hat would not be parenthetical, as it is in the first sentence.

THAT SAID, the second sentence would still be correct in a context where the blue hat didn't matter. In that case, you're right, it would just be a style choice which version of that sentence to use.

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