"That" vs. "Which"

I know, this is hardly science. However, the only reason this comes up is that I've been editing papers recently.

Microsoft Word has a number of hang-ups that can be described as "cute" when you're editing a page-long document, and something considerably stronger when you have a 10-page scientific paper you're trying to spellcheck/format. One of these is the dreaded "that"/"which" dilemma, which seems to haunt it constantly.

As far as Word's grammar-checker is considered, there are two mortal sins surrounding the words "that" and "which":

1. Thou shalt not have a "which" unless preceded by a comma.
2. Thou shalt not have a "that" directly after a comma.

And I always found these rules incredibly arbitrary, like splitting infinitives or not starting sentences with "and"^[1]. Today I faced this issue in Word again, and instead of either submitting to the product's directives or assuming that the MS Word development team were out to get me, I thought I would look into the issue.

My first stop was the venerable Strunk & White. Turns out the book has a whole section on the conundrum, in which it states:

That is the defining, or restrictive, pronoun, which is that nondefining, or nonrestrictive.

The lawn mower that is broken is in the garage. (Tells us which one.)

The lawn mower, which is broken, is in the garage. (Adds a fact about the only mower in question.)

Strunk & White also suggest placing parenthetical statements between commas, and further state: "Nonrestrictive relative clauses are parenthetic" (i.e. almost every "which" clause will be parenthetic, and will thus require commas). However on the subject of parenthetic clauses they also state:

This rule [of using commas] is difficult to apply; it is frequently hard to decide whether a single word, such as however, or a brief phrase is or is not parenthetic. If the interruption to the flow of the sentence is but slight, the commas may be safely omitted.

This little sentence gives us an out, assuming we wish to abide by Strunk & White's rules. As long as your clause in a) nonrestrictive (i.e. it doesn't help us work out which item we're talking about here) and b) doesn't impede the flow of the sentence, the comma is optional. But this, it seems, is the only time you can justify using "which" without also inserting Word's favourite comma.

Fowler's Modern English also demonstrates an exception to this rule: clauses preceded by a preposition. You can put a preposition in front of "which" (e.g., "in which", "from which", "by which"), but do the same thing with "that" and you get a mess of language. Sometimes you can shove the preposition to the end of the sentence:

My father is rebuilding the pool that I keep my eels in.

Or, you can swap it out for "which", which has no problem with those pesky prepositions:

My father is rebuilding the pool in which I keep my eels.

Both of these are fine, and in fact Word will happily accept the second example as a valid sentence, despite the lack of comma.

So in a roundabout way, we reach a conclusion: Word's recommendations on the use of "that" and "which" are in fact literary best practice in almost all cases. If you find yourself railing against Word's seemingly-arbitrary guidelines about the usage of "that" and "which", it might be worth checking to see if you're using them correctly.

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1. See what I did there. ↩︎