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That or which?

Writing a sentence involves using a combination of restrictive and nonrestrictive clauses. Restrictive clauses are essential to the meaning of the sentence, whereas nonrestrictive clauses contribute additional information. For example, in the sentence, “the book by Jones that is red is my favourite”, the clause “that is red” is a restrictive clause because it identifies that amongst the books written by Jones, it is the red book that is the favourite. However, in the sentence, “the book by Jones, which is red, is my favourite”, “which is red” is a nonrestrictive clause because it adds additional, non-critical information to the description. The sentence could exist without the nonrestrictive clause: “The book by Jones is my favourite”.
 
When you’re deciding whether to use “which” or “that” in your sentence, consider if you’re writing a restrictive or nonrestrictive clause. Is the information that follows the which/that critical to the meaning of the sentence? If yes, use “that”; if not, use “which”. For more information, please refer to "Who or whom? That or which?" or the Grammar Girl's "Which versus that".
  
One final punctuation point: when placing a nonrestrictive clause in a sentence, the clause should be contained by commas: “The book by Jones, which is red, is my favourite”. The commas at the beginning and end of the clause indicate where the non-restrictive clause starts and finishes, thereby alerting your reader to the essential/non-essential information in the sentence.

Theresa Bell
Writing centre coordinator 

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