The use of “that” versus “which”
Ex. 1. The book that I need is on the shelf.
2. The book, which I need, is on the shelf.
There is a subtle butimportant difference between the use of that and which in a sentence and it has to do primarily with relevance.
Grammarians often use the terms "restrictive" and "non-restrictive" whenit comes to relative clauses. A relative clause provides additional information about the noun it describes, but it may be considered relevant or irrelevant to the overall point ofthe sentence. In other words, a restrictive relative clause, which often begins with that, is usually considered essential or restrictive. On the other hand, relative clauses beginningwith which may contain non-essential information and would be considered non-restrictive.
Therefore, in the sentence:
• “The book that I need is on the shelf”, that isrelative to book as there may be hundreds of books on the shelves, and so it is important to know the book I need is specifically on the shelf….
In that sense, the relative clausebeginning with that would be considered restrictive, since it is an essential piece of information that identifies the book.
• “the book, which I need, is on the shelf”,
“In thissentence, the relative clause 'which I need' is separated by commas and is not essential to the main idea of the sentence. It could be removed and the sentence would still makesense. If the relative clause can be removed without changing the sentence's meaning, it would be considered non-restrictive. Anyone can identify the specific subject, in this case thebook, so "which" would be the correct pronoun to use. "Which” is almost always used to set off a non-restrictive relative clause, which you may remember as a parenthetical phrase?