That, Which, and That Which
Rules of Thumb
You’ll encounter two schools of thought on this point. First are those who don’t care about any distinction between these words, who think that “which” is more formal than “that,” and who point to many historical examples of copious “whiches.” They say that modern usage is a muddle. Second are those who insist that both words have useful functions that ought to be separated, and who observe the distinction rigorously in their own writing. They view departures from this distinction as “mistakes.”1
I am a proud member of the former group. Here is how the words are most frequently used by those who know what they are doing:
- which is only used in non-restrictive clauses, which are always encapsulated by commas.
- that is prepended to restrictive clauses: chunks of information that are essential for the meangings of their respective sentences.
- that which can usually be replaced by what. If it cannot, it is preferable to use that which rather than the technically-valid but cumbersome that that.
If you follow these rules, you will find yourself writing few which-es and rather a lot of that-s. If you do a lot of formal writing, you might worry that this will mark you as a philistine. Have some courage, and rest assured that you are in the right company.
It should be noted, however, that Garner’s advice, which I generally follow, applies in this case mainly to American English. British English is far sloppier with the distinction, often using which without encapsulating commas—that is, in situations where Garner’s rules would urge the opposite. I suspect that this is why we tend to think of which as the more formal of the two. I also suspect that this comes from Latin, which does not have separate relative pronouns for restrictive and non-restrictive clauses (qui/quae/quod may be either which or that).
On descriptive grounds, Garner’s advice fails for British English. But on prescriptive grounds, it applies to all English speakers: it is useful, if not quite necessary, to use separate words for restrictive and non-restrictive pronouns.