Confusion in identifying defining and non-defining relative clauses

The words used to describe these kinds of clauses vary a lot: I like Descriptive and Defining.

American English has a tendency to use “that” to introduce defining clauses, and “which” or “who” (always preceded by a comma) for descriptive clauses – I think this is a good idea – but it is not a “rule”. (In British English, this tendency is not so marked.)

As general guidance,

1. When written, descriptive clauses are always off-set by commas.

2. When written, defining clauses are never off-set by commas.

3. Descriptive clauses can always be omitted because they give unimportant or unnecessary information – information that is an aside or a casual remark.

4. Defining clauses cannot be omitted. Defining clauses tell you which precise item is being referred to.

For example:

1 Wife to her husband: “The cat that I saw yesterday is in the garden again.” – “The cat that I saw yesterday” is a single noun phrase acting as the subject, and “that I saw yesterday” defines the cat – it tells you which cat it was – it was the cat that she had seen yesterday and no other cat.

If the wife had omitted that I saw yesterday, and said “The cat is in the garden again”, her husband would have said “Which cat are you talking about? You have never mentioned a cat to me!”

2 Wife to her husband: “The cat , which I saw yesterday , is in the garden again.” = “The cat (and, by the way, I also happened to see it yesterday,) is in the garden again.” Also, the sentence assumes that her husband knows which cat it is.

If the wife had omitted , which I saw yesterday, and said “The cat is in the garden again”, it means that she is aware that her husband already knows which cat she was talking about.

Thus:

"I saw a car which was white."

This would be a defining clause - there is no comma - and it might be a reply to "There are no white cars in this area."

"She went into the apartment, which had blue walls."

This would be a descriptive clause - there is a comma (and "which" has been used) - It means "She went into the apartment(that had been mentioned early) and, by the way, the apartment just happened to have blue walls, but they could have been any colour and it would not have made any difference."

First, I don't see why this speaker speaks of a sentence as being a clause: that makes no sense. Then, the terms apply to the relative clauses, strictly, and on top of that they are not "definite" nor "indefinite". On the one hand they can be called according to alternative terminologies

•            restrictive                    /identifying          /defining        /determinative/integrated and on the other
• descriptive/non-restrictive/non-identifying/non-defining/appositive       /supplementary (ref).
  The term "definite clause" seems to apply only to the domain of logic and programming (Wikipedia, wiktionary).

The first clause ( which was white) is a case of restrictive clause (no commas to set it apart) but this is so in BrE only. In AmE, rather formal, you use only "that" for restrictive clauses (ref.1, ref. 2).

The clause "which had blue Walls" is non-restrictive (a comma is used to set it apart from the other clause). It is not a defining clause or according to the usual terminology it is not an identifying clause: it does not identify the apartment. Here is why: you can eliminate "which had blue walls".

• She went into the apartment.

In saying so you do not communicate any vague information, you are still referring, in the given context, to a unique apartment ; the main clause still stands correct on its own. (She went into the apartment, which happened to have blue walls.)

If the sentence had been "She went into the apartment which had blue walls." (no comma, pay attention to this difference), then suppressing the clause, you communicate imprecise information because she went into that apartment that had blue walls and not into another one; so, the apartment is not identified any more; for instance you can ask "Was it the blue walled one or was it the white walled one?".