"To this day I cannot be sure when I should use ‘that’ and when I should use ‘which,’ but my secretary knows, and between us we keep up some sort of pretence." So admitted the venerable Robertson Davies in 1978.

Davies is just one of many authors—not to mention ordinary, non-literary folk—to confess themselves befuddled by these deceptively innocuous pronouns. Little wonder, when language experts themselves can’t agree. Over the past century, few topics have ruffled more feathers in the linguistic aviary than that and which, with language pundits squabbling over whether the distinction is real or bogus, enforceable or laughable.

The convention

Leaving for a moment the rich historical debate, let’s look at the that/which rule—or convention, as we might more accurately call it. On the surface it’s relatively simple: use that to begin a restrictive clause and which to begin a non-restrictive clause. But in reality, many writers have as much difficulty differentiating between restrictive and non-restrictive clauses as they do sorting out the words that should begin them.

A non-restrictive clause describes a word without restricting or limiting its meaning. It helps to think of a non-restrictive clause as non-essential. It interrupts the main point of the sentence, adding extra, "by the way" information that is interesting but not entirely necessary. Here are some examples:

• Shaun’s father, who was a tax accountant for thirty years, now runs a psychic hotline from his cottage in the Muskokas.
• Even now I miss Hillsdale Elementary, where the most valuable lessons unfolded in the schoolyard.
• Lunch, which was served at 12:30 sharp, consisted of codfish hash, beet greens and boiled tea.

Notice that in all three cases, the non-restrictive clause can disappear from the sentence without affecting the main idea. To signal that a non-restrictive clause is merely an interruption or elaboration, we set it off with a pair of commas—or one comma if, as in the second example, the clause is at the end of the sentence.

Notice also that the clause in the third example begins with which, not that, since which is the right pronoun to begin a non-restrictive clause.

A restrictive clause is just the opposite. It restricts or limits the meaning of the word or words it describes. No mere interruption or interjection, it provides information that is essential to the main idea. A restrictive clause often defines what it describes, as in these examples:

• Vancouver residents who do not hold secure, well-paying jobs must resent the city’s portrayal as a land of opportunity.
• We cherish warm memories of our fellow alumni who are dead.
• Years ago I painted a picture of the deserted warehouse that burned down last week.

In all three sentences, the restrictive clause is essential to the main meaning, defining which Vancouver residents must feel resentful, which fellow alumni evoke warm memories, which deserted warehouse I painted a picture of. To show that the clause is essential, we run it into the sentence without commas.

Punctuation skeptics may snicker, but the presence or absence of commas around descriptive clauses is not some finicky bit of window-dressing, nice for the fanatical few but hardly necessary for the rest of us. On the contrary, commas (Commas That Count and Commas That Clutter) can radically affect meaning.

Imagine commas around the restrictive clause in the first sample sentence above. Suddenly the main idea becomes "Vancouver residents (all of them)—who, by the way, do not hold secure, well-paying jobs—must resent the city’s portrayal as a land of opportunity." A comma before the clause in the second sentence wreaks similar havoc with the sense. Now the main idea is "We cherish warm memories of our fellow alumni—who, sorry to say, are all dead."

Notice that in the third sentence, the restrictive clause begins with that, not which. Those who observe a distinction between the two reserve that for restrictive clauses and which, as we’ve seen, for non-restrictive.

The controversy

If that’s all there is to that and which, then why is this modest principle the subject of so much grammatical hand-wringing?

The which part of the convention is largely beyond dispute. All reputable language guides and authorities stipulate that which must begin a non-restrictive clause. The disagreement centres on that. The fact is that to many ears, "Years ago I painted a picture of the deserted warehouse which burned down last week" sounds just as correct and natural as the that version. Countless people, including professional—indeed, award-winning—writers, use which and that interchangeably to begin restrictive clauses, basing their choice more on sound and rhythm than on an ironclad rule. And they’ve done so for centuries.

It’s difficult to pin down exactly when the that/which question took root in English grammar. Some point to H.W. Fowler as the inventor of the rule. After all, he wrote about it in 1906 in The King’s English, and later in his hugely influential Modern English Usage (1926). But it’s probably more accurate to say that Fowler popularized the rule, since language scholars before him had advocated the same notion.

Regardless of its provenance, the that/which distinction has attracted stalwart supporters and steadfast foes. With two factions to choose from, what’s a person to do?

The bottom line

Bear in mind that the one incontrovertible rule concerns the punctuation: commas with a non-restrictive clause, no commas with a restrictive. If a clause is non-restrictive (therefore preceded by a comma), use which rather than that. Remember, there’s no debate there. If a clause is restrictive (with no commas), you can choose sides. If you want to differentiate, use that. If not, use whichever pronoun sounds better.

That said, in casting your support, you should know that most Canadian grammar and usage texts advocate the that/which distinction, particularly for formal writing. Most Canadian editors follow the convention, as do many careful writers. So if you decide to go that and which hunting, you’ll find yourself in distinguished company.

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