Are You Using “That” and “Which” Wrong? Probably.

If you’re like many writers, the words that and which probably seem interchangeable—or at least tricky to use correctly.

But mixing them up can subtly change your meaning, confuse readers, and make your sentences clunky or unclear. Even experienced writers struggle with this one!

In this article, we’ll break down the key difference between that and which, show you easy ways to spot mistakes, and explain when it’s okay to bend the rules.

By the end, you’ll have a simple, reliable trick to make sure you’re using these words confidently—and making your writing clearer in the process.

What’s the Difference Between “That” and “Which”?

The core difference lies in the kind of information these words introduce in a sentence.

• That introduces an essential or restrictive clause — information necessary to the meaning of the sentence. Without this clause, the sentence’s meaning changes or becomes unclear.
• Which introduces a nonessential or nonrestrictive clause — additional information that can be removed without changing the basic meaning of the sentence. These clauses are set off by commas.

Examples:

• Books that have red covers are on the top shelf.
  (Only the books with red covers are on the top shelf — this clause is essential.)
• My book, which has a red cover, is on the top shelf.
  (I have one book, and as extra info, it has a red cover — this clause is nonessential.)

Examples – When to Use “That” vs. “Which”

Understanding the difference is easier when you see side-by-side examples.

Key points:

• Use that to specify or limit what you’re talking about.
• Use which to add extra details or commentary.
• Remember: which clauses need commas; that clauses do not.

Why This Mistake Slips Through So Often

Many writers confuse that and which because spoken English rarely follows the strict rule. In everyday conversation, people often use them interchangeably, so the distinction feels subtle or even unnecessary.

Additionally, grammar checkers and automated tools often don’t flag misuse of that and which, since both are grammatically correct words used in clauses.

Writers also tend to rely on what sounds right, rather than on the formal rule, especially when writing quickly or editing late at night.

The result? Sentences with unclear meaning, awkward flow, or missing punctuation—which can confuse readers or make writing seem less polished.

How to Spot and Fix the Mistake in Your Writing

Fixing your that vs. which usage starts with understanding whether the clause is essential or nonessential.

Ask yourself:

• Is this information crucial to the meaning of the sentence?
• Can the sentence stand alone without this clause?

If the clause is essential—meaning the sentence’s meaning changes or becomes unclear without it—use that and do not use commas.

If the clause is extra information that can be removed without changing the sentence’s core meaning, use which and set it off with commas.

A simple way to decide between that and which:

• Remove the clause starting with that or which from the sentence.
• If removing the clause changes the meaning or makes the sentence unclear, the clause is essential, so use that (no commas).
• If removing the clause doesn’t change the core meaning and the sentence still makes sense, the clause is nonessential, so use which (with commas).

Examples:

• The car that is red is mine.
  (Remove that is red → The car is mine. Meaning changes—could be any car—so that is correct.)
• My car, which is red, is mine.
  (Remove which is red → My car is mine. Sentence still makes sense, so which with commas is correct.)

Quick tips:

• Look for commas—if there’s a comma before which, you’re probably correct.
• When in doubt, try the cut test!
• Reading sentences aloud can help catch awkward phrasing.

Bonus – When It Doesn’t Matter (and When It Really Does)

While the that vs. which rule is important for clear, formal writing, there are times when strict adherence isn’t necessary.

• In casual writing and fiction: Writers often bend or break the rule to maintain flow or match natural speech. Readers usually won’t mind or even notice.
• In technical, legal, or academic writing: Precision is key. Misusing that and which can cause confusion or change the meaning, so it’s best to follow the rule strictly.

Remember: The goal is clarity. If your sentence is clear and your reader won’t be confused, the rule can be more flexible.

Using that and which correctly can sharpen your writing and prevent confusion, but it’s a rule many writers struggle with. Remember, that introduces essential information without commas, while which adds nonessential details set off by commas.

When in doubt, try the “cut test”: if removing the clause changes the meaning, use that; if not, use which.

And keep in mind, in casual writing, the rule can bend a little—but clarity should always be your top priority.

With these tips, you’ll write with more confidence and precision, making your prose clearer and more polished.