"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:

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:

(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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