What's your biggest editing/proofreading peeve?
A few of them are really annoying, such as the overuse of "of" with another preposition, for example. But the one that really drives me up the wall is the misuse of THAT and WHICH.
For the curious, however, the relative pronoun that is restrictive, which means it tells you a necessary piece of information about its antecedent: for example, “The word processor that is used most often is WordPerfect.” Here the that phrase answers an important question: which of the many word processors are we talking about? And the answer is the one that is used most often.
Which is non-restrictive: it does not limit the word it refers to. An example is “Penn's ID center, which is called CUPID, has been successful so far.” Here that is unnecessary: the which does not tell us which of Penn's many ID centers we're considering; it simply provides an extra piece of information about the plan we're already discussing. “Penn's ID Center” tells us all we really need to know to identify it.
It boils down to this: if you can tell which thing is being discussed without the which or that clause, use which; if you can't, use that.
There are two rules of thumb you can keep in mind. First, if the phrase needs a comma, you probably mean which. Since “Penn's ID center” calls for a comma, we would not say “Penn's ID Center, that is called CUPID.”
Another way to keep them straight is to imagine by the way following every which: “Penn's ID center, which (by the way) is called CUPID. . . .” The which adds a useful, but not grammatically necessary, piece of information. On the other hand, we wouldn't say “The word processor which (by the way) is used most often is WordPerfect,” because the word processor on its own isn't enough information — which word processor?