What’s the difference between which and that? Is it an important difference, or do only stuffy academic types insist that the two words shouldn’t be used interchangeably? (If you’re British or writing for a British audience, there really is no distinction, so you can stop reading now.)

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In marketing we often choose to overlook certain grammar rules in favor of friendly and engaging copy, which is a smart move. We’re not doing our jobs if we’re not speaking the same language as our target audience. Factor in the Internet Age’s shrinking attention spans, and it’s especially important to get to the point — or someone else will beat us to the punch.

But I’m a copy editor, one of Hanley Wood Marketing’s “copy cops” (#copycop on Twitter). While I can let some things go, it’s still my job — and even in my nature — to defend proper grammar. I must choose my battles wisely, which means I’m that much more committed to emerging victorious.

The which vs. that debate is one of those battles. I will always mark these words when they aren’t used according to Chicago Manual of Style or the Associated Press Stylebook, even though the usage notes in dictionaries tend to allow more wiggle room.

So what do these esteemed style guides have to say? Here are some examples straight from the sources:

AP, Chicago Weigh In

Let’s look first at AP style, which is used more often in marketing copy:

“Use that for essential clauses, important to the meaning of the sentence, and without commas: I remember the day that we met. Use which for nonessential clauses, where the pronoun is less necessary, and use commas: The team, which finished last a year ago, is in first place. (Tip: If you can drop the clause and not lose the meaning of the sentence, use which; otherwise, use that. A which clause is surrounded by commas; no commas are used with that clauses.)”

I love AP’s tip, and I think it’s especially helpful for writers who aren’t daily immersed in grammar matters. It’s a quick way to decide which word to use, and it doesn’t require tracking down a long explanation. Ask yourself, Would I put commas around this phrase? If not, use that. Does the sentence still make sense if I were to remove this clause? If yes, use which.

And here’s an excerpt from Chicago Manual of Style:

“These are both relative pronouns. In polished American prose, that is used restrictively to narrow a category or identify a particular item being talked about (any building that is taller must be outside the state); which is used nonrestrictively—not to narrow a class or identify a particular item but to add something about an item already identified (alongside the officer trotted a toy poodle, which is hardly a typical police dog). Which should be used restrictively only when it is preceded by a preposition (the situation in which we find ourselves).”

You may have noticed the qualifier “polished American prose.” While consumers aren’t likely to mistake Facebook updates and Twitter posts, or even magazine articles, for fine literature, using which and that properly adds the type of clarity that allows them to quickly receive your message.

The way I see it, even in marketing copy, where our goal is to be creative, compelling, and punchy, our reputation is on the line, and grammar still matters. We can be looser, for sure, and we can bend the rules. Sometimes we can even break them. But there’s breaking rules on purpose, and then there’s accidentally breaking a rule and confusing our audience with a garbled sentence.

When in doubt, look it up, and try to save your own battles for when you want to be especially clever and grammatically rebellious.