An essential, or restrictive, clause is a part of a sentence that provides integral context for the reader. A nonessential, or nonrestrictive, clause is parenthetical, presenting additional information that is not necessary for reader comprehension. Writers are, with increasing frequency, neglecting to appreciate the distinction, thereby undermining the effectiveness of the language they use. The following sentences illustrate how this carelessness (or ignorance) adversely affects meaning. Discussion and revision of each sentence responds to the fact that each of the following sentences erroneously treats optional material as required content.
1. Phyllis Schlafly, the conservative political activist who devoted much of her energy in the 1970s to stopping the passage of the Equal Rights Amendment that would have banned discrimination on the basis of sex in the United States, has died.
The phrase “The Equal Rights Amendment that would have banned discrimination . . .” implies that more than one such amendment was proposed and that this one in particular, and no others, was intended to ban the specified discrimination. However, “would have banned . . . United States” is helpful but nonessential information about the one and only amendment, and that phrase should be included parenthetically: “Phyllis Schlafly, the conservative political activist who devoted much of her energy in the 1970s to stopping the passage of the Equal Rights Amendment, which would have banned discrimination on the basis of sex in the United States, has died.”
2. Smith was referring to the Columbian mammoth which roamed the continent of North America before migrating to the Channel Islands.
The sentence seems to imply that more than one species of Columbian mammoth existed, and that the one in question was native to North America but ended up on adjacent islands (with which substituting for the preferred pronoun that). However, the phrase “which roamed the continent of North America before migrating to the Channel Islands” is additional information attached to the main clause, “Smith was referring to the Columbian mammoth,” so it should be set off with a comma: “Smith was referring to the Columbian mammoth, which roamed the continent of North America before migrating to the Channel Islands.”
3. The two dwarf planets are in the asteroid belt that’s littered with rocky debris from the formation of the sun and planets some 4.5 billion years ago.
The implication here is that more than one asteroid belt exists, and the one under discussion is littered with the described rocky debris; one or more others, presumably, has no such detritus. But the part of the sentence that follows “asteroid belt” is nonessential information that should be appended to the main clause parenthetically, with a comma separating the two sentence segments: “The two dwarf planets are in the asteroid belt, which is littered with rocky debris from the formation of the sun and planets some 4.5 billion years ago.”
The revision to #1 is technically perfect, including the that/which switch.
Could also do this:
Phyllis Schlafly has died. The conservative political activist devoted much of her energy in the 1970s to stopping the passage of the Equal Rights Amendment, which would have banned discrimination on the basis of sex in the United States, has died.
But then the exercise would be pointless.
Strunk & White, at least the last time I checked, and I’ve followed their advice steadfastly since I was a student, always have the non-restrictive clause start with a “which”, and always set if off with commas; the restrictive always starts with “that”. However, I’ve noticed that this rule is not followed closely in British English, where “that/which” seem to be used interchangeably, as they were in Ontario when I was learning English. Any comment?