Non-restrictive relative clauses must always be introduced by which and never by that.

So can I say something like

  • Both my computer and my car, that is run-down, were quite cheap.

An Oxford dictionary seems to say no. But I think that the non-restrictive phrase "is run down" is non-defining because I only have one car, and that it works with 'that'.

  • Comments are not for extended discussion; this conversation has been moved to chat. – tchrist Aug 25 '18 at 23:35

If a native English speaker says my car that is run down, (in one breath group, so written without a comma), other native English speakers will probably conclude that the speaker has more than one car, and is identifying one of them. They could have substituted which for that, but that is less common, and a bit formal.

If they started a new breath group after "car", with the corresponding intonation - as represented by a comma - followed by "which", (my car, which is run down) the natural meaning of this is that they have only one car, and are commenting that it is run down. My car, that is run down is anomalous, and I can't think of a context in which somebody would say it.

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Chicago manual of style 5.220

Which should be used restrictively only when it is preceded by a preposition {the situation in which we find ourselves}. Otherwise, it is almost always preceded by a comma, a parenthesis, or a dash. In British English, writers and editors seldom observe the distinction between the two words.

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  • The Wikipedia article on commas is interesting when it talks about comma use and independent clauses. Basically it says that some style guides require a comma before a coordinating conjunction separating two independent clauses, so "Mary walked to the party, but she was unable to walk home" has to have a comma, but there is no comma where the second clause is dependent, such as "Mary walked to the party but was unable to walk home". It states that some style guides prescribe this. Both the Chicago Style Manual and Oxford Style Guide do. en.wikipedia.org/wiki/Comma#Separation_of_clauses – Zebrafish Aug 25 '18 at 3:39
  • @Zebrafish yes difficult to know when smt is stylistic or grammatical, if that's what you mean? style guides are mroe approachable tho – concerned Aug 25 '18 at 3:51

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