Even the dictionary citation illustrating a “restrictive clause” is a bummer: Webster’s gives the example of “that you ordered” in the sentence “The book that you ordered is out of print.” Oh, no! The Random House College Dictionary has a slightly more positive definition for the grammatical sense of restrictive—“of or pertaining to a word, phrase, or clause that identifies or limits the meaning of a modified element”—but it goes on to give yet another bummer of an example: “that just ended in The year that just ended was bad for crops.” Just my luck: the book that I wanted is out of print, and now the price of corn is going to skyrocket.
I don’t mean to make this any more confusing than it already is, but let’s not pretend it’s easy. Here is the Random House College Dictionary on nonrestrictive: “pertaining to a word, phrase, or clause that describes or supplements a modified element but is not essential in establishing its identity, as the relative clause which has been dry in the sentence This year, which has been dry, was bad for crops. In English, a nonrestrictive clause is usu. set off by commas.” This is admirably clear, but we’re still suffering from a drought.
Do you see the difference between “The year that just ended was bad for crops” and “This year, which has been dry, was bad for crops”? With “this year,” we already know which year the writer is talking about: you could write with perfect clarity, “This year was bad for crops.” You could also write, “The year was bad for crops,” but in the context you’d need to know what year was meant; adding “that just ended” identifies the year.
“Nonrestrictive” has a nicer ring to it than “restrictive.” I like nonrestrictive clothing and a nonrestrictive diet. Here is the definition from Web II of a nonrestrictive clause: “An adjective clause which adds information but is so loosely attached to its noun as to be not essential to the definiteness of the noun’s meaning (the aldermen, who were present, assented)—called also descriptive clause. Such a clause is marked off by commas, whereas the corresponding restrictive clause is not (the aldermen who were present assented = such aldermen as were present assented).” Of course, the hilarious thing about this is that the definition itself uses “which” (“an adjective clause which adds information”) where standard modern American usage prefers “that”: “an adjective clause that adds information.”
I always have to pause and think what I mean by restrictive: you think something that is restrictive is going to take the commas, that the commas will restrict the clause, cordon it off, keep it out of the way. But it is just the opposite: a restrictive clause is so much a part of the noun it modifies that it doesn’t need any punctuation to stake its claim. The original purpose of a comma was to separate, and a restrictive clause does not want to be separate from what it modifies: it wants to be one with it, to be essential to it, to identify with it totally. Once you get the idea of what is restrictive (She was a graduate of a school that had very high standards), everything else is nonrestrictive (He graduated from another school, which would admit anyone with a pulse).