One of the most common mistakes I see among new and veteran writers is using “that” and “which” incorrectly. Don’t feel bad if you don’t know when each is called for or why they’re not interchangeable. Grammar isn’t in everyone’s wheelhouse.
Essential clauses
The first step is to grasp essential and nonessential clauses, often referred to as restrictive and nonrestrictive clauses. You can’t remove an essential clause from a sentence without changing the meaning of that sentence. That’s why it’s considered essential. “That” introduces an essential clause.
Here are two examples:
I remember the day that we developed our highest-performing content marketing plan.
This sentence wouldn’t make any sense if I wrote, “I remember the day which we developed our highest-performing marketing plan.”
My CEO said that our marketing KPIs have improved so much this year that she’s going to increase our budget.
You’d get some strange reactions if you wrote, “My CEO said which our marketing KPIs have improved so much this year which she’s going to increase our budget.”
Nonessential clauses
In contrast, you can delete a nonessential clause from a sentence without changing the meaning of that sentence. Think of a nonessential clause as a way to elaborate. “Which” introduces a nonessential clause, as in these examples:
The redesigned packaging, which features eco-friendly materials, aligns with our commitment to sustainability.
If you delete the nonessential clause, “which features eco-friendly materials,” the rest of the sentence stands on its own.
Our new product line, which will go on the market next week, will surprise our competitors.
The important information here is that the company’s new product line will shake up the competition. The nonessential clause, “which will go on the market next week,” provides additional details. A comma always precedes a nonessential clause.
Which is correct?
A good way to understand when to use “that” and “which” is to compare the same sentence with both words and see how the meaning changes. Here’s an example:
The marketing book that has insights from 10 industry leaders is available at two bookstores near my house.
If two bookstores near my house have multiple marketing books, I’d use “that” to specify the one that has insights from industry leaders.
The marketing book, which has insights from 10 industry leaders, is available at two bookstores near my house.
If only two bookstores have the marketing book that has insights from 10 industry leaders, the clause is nonessential, and I’d use “which.”
Need more help?
Are you up to speed on that “that” versus “which” now? If you still have questions, check out Grammar Girl’s “Quick and Dirty Tips” 554 podcast. Mignon Fogarty has covered grammar for many years and is an excellent resource.
