"Which" vs. "that"
Use that for restrictive clauses (i.e., restricting what is referred to) and which for non-restrictive clauses. The acid test is: if you can remove the clause and it still makes sense, use which, otherwise use that.
Wrong: "This is shown in the following method that is invoked by the interpreter."
Right: "This is shown in the following method, which is invoked by the interpreter."
(You can remove the whole "which ..." and it makes sense.)
Wrong: "We call the equivalent parts which describe the same feature `building blocks'."
Right: "We call the equivalent parts that describe the same feature `building blocks'."
(You cannot remove the qualifying text.)
Note that many authors confuse "which" and "that", but there is a clear semantic difference. Consider the two following statements:
- You should write a PhD thesis, which will further your academic career.
- You should write a PhD thesis that will further your academic career.
Both are grammatically correct, but the first states that writing any PhD thesis will further your career, whereas the second suggests that you should write a particular kind of thesis that will be good for your career. Note that a comma is needed to introduce the non-restrictive clause.
Caveat: Actually it is common in English to use "which" instead of "that" also for restrictive clauses, as in:
- You should write a PhD thesis which will further your academic career.
(Note the absence of the comma!) In many cases this will cause no confusion, but wherever it might (like here), it makes good sense to maintain a clear distinction and use "which" only for non-restrictive clauses.
See also: Which or That?
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