That vs. Which: Yes, There Is a Difference

 

That or which? There is a difference, one worth observing. Well, not everyone agrees. Some say, who cares? After all, historically, that and which have often been used interchangeably. But I’m among those who recognize that each word has a distinct and useful function.

Grammar and usage expert Brian Garner (he’s as big as it gets in grammar) writes of these two groups, “Those in the first probably don’t write very well; those in the second just might.” That’s because writers who distinguish between that and which know and observe the difference between restrictive and nonrestrictive information (which I’ll explain below). As a result, their writing has more clarity. I say you can’t argue with that.

The rule

That introduces restrictive information and does not take a comma. Which introduces nonrestrictive information and takes a comma.

Restrictive information is essential, defining information. The meaning of the sentence wouldn’t be the same without it. You would never put it in parentheses (or put a comma on either side, which is pretty much the same thing).

Nonrestrictive information is nonessential, supplemental information. If you got rid of it, the sentence would retain its basic meaning. It’s parenthetical, an aside, so you put a comma on either side to set it off. The commas say, here’s the beginning of the bit that doesn’t matter so much, and here’s the end of it.

The most common mistake is to use which when you need that, so pay special attention to which in your writing.

Example

The difference between these two sentences . . .

All the lamps that were sold before 2008 were recalled.
All the lamps, which were sold before 2008, were recalled.

. . . is the same as the difference between these two:

All the lamps that were sold before 2008 were recalled.
All the lamps were recalled.

In the first sentence, that signals that the clause with the date is restrictive, essential. Only the lamps sold before 2008 were recalled. Any sold after were not.

In the second sentence, which signals that the clause with the date is nonrestrictive, or supplemental. All the lamps we’re talking about were recalled, and by the way, all these dud lamps were sold before 2008.

Big difference.

Example

Polar bears that eat humans are best experienced at a distance.
Polar bears, which eat humans, are best experienced at a distance.

Sentence one is telling us that only the man-eating polar bears need to be kept at a distance, suggesting that the normal bears would turn up their nose at fresh human.

Sentence two is telling us that all polar bears eat humans and therefore all should be kept at a distance. It also illustrates the fact that supplemental “which” information can be compelling. Still, if you cut that information, the point is made: keep your distance.

And which sentence is true and correct? Number two! A man-eating polar bear is a normal polar bear.

Really big difference.

While your head is in restrictive/nonrestrictive mode, check out my very first post on this blog: “Restrictive Appositives: A Bad Case of Comma Confusion.” It’s as relevant as ever.