I hesitate between which and that after because:

I prefer orange cars because blue cars would be thought of as the police(,) which/that is always aggressive.

Is it a restrictive/non-restrictive issue here?

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    That example... am I the only one it strikes as ungrammatical? be though of as the police works, but I'm trying to think of alternate intentions. – jimsug May 12 '14 at 9:54
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    Not related to the main question, but "which/that is always aggressive." refers to the police? Or blue cars? My wondering is why its singular "is". are sounds better to me. – user3169 May 12 '14 at 21:24
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    I think you mean basically: I prefer orange cars to blue cars, since blue cars can be mistaken for police cars and the police are always aggressive." ??? – CocoPop Aug 10 '14 at 12:28

There are two issues here:

  • The relative clause is non-restrictive clause, and only wh- forms may head non-restrictive relative clauses. Both wh- and that may head restrictive relative clauses. In restrictive clauses, but not non-restrictive clauses, the relativizer may be omitted if it does not stand for the subject of the relative clause (as it does in your example).

  • At least in US usage, the police is considered a body of people and referred to with who rather which, so if you wish to say that the police are always aggressive you should use who.

    I prefer orange cars because blue cars would be thought of as the police, who are always aggressive.

It may be, however, that what you are trying to say is that the use of blue cars arouses thoughts of aggressiveness, just as it arouses thoughts of the police. It is true that a which clause in this position leads the reader to think that the referent of which is the entire preceding clause, blue cars would be thought of as the police. But it is not idiomatic English to say that this thought is always aggressive—that would mean that the thought was threatening to attack you! What you should say in this case is something more like

I prefer orange cars because blue cars would be thought of as the police, which always suggests aggressiveness.


'which' is used before non-restrictive clause whereas 'that' is used before restrictive clause. Restrictive clause forms unavoided part of the sentence which cannot be left off and non-restrictive clauses are avoidable clauses which cannot be peeled off the sentence. Here police which normally is considered aggressive makes clause 'always aggressive' non-mandatory so you can use 'which' in that senetence.

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