Which is correct?
EINSTEIN is the product of government projects ______ were undertaken to improve U.S. government services on the Internet.
The general rule here is to use that, with no comma preceding it, if the adjective clause following is essential to the sentence. Here, the fact that the projects were undertaken to improve government services is the point of the sentence. If we wanted to introduce a little tidbit of inessential information, we could use which with a comma preceding it:
Einstein is the product of government projects, which were conducted over a period of three years.
If you follow AP style, that’s the rule. However, rules change and this one seems to be a case in point.
Here’s an example from World Wide Words:
Another cause of stress is a traumatic event that is out of the ordinary and has a major impact on the person’s life.
The argument here is that the clause “that is out of the ordinary and has a major impact on the person’s life” modifies and constrains “event”. It’s not just any event but one specific type of event, to the extent that the whole block from “event” onwards forms one idea. The clause is restrictive.
Older grammar books make two firm points about the difference between the two types of clause:
* Restrictive clauses are introduced by that and are not separated from the rest of the sentence by commas.
* Non-restrictive clauses are introduced by which and must be separated by commas from the rest of the sentence to indicate parenthesis.
Author Michael Quinion goes on to discuss the way those rules are changing and when you can — and can’t — bend the rules. His conclusion:
If you wish to write naturally, don’t fuss too much about the usage of that versus which. Obsessive correction (sarcastically called a which hunt) is best avoided. If your sense of the language is not strong enough to be sure of the right pronoun, use that for the restrictive cases and which for the others and you won’t go wrong.
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