Which vs. That: The Difference in Plain Terms
Most which vs. that confusion comes down to one question: is the clause essential to the meaning of the sentence, or is it extra information? If the clause is essential, use that. If it is extra and you set it off with commas, use which. That single distinction handles the large majority of cases in American English.
The Core Rule
A clause starting with that restricts the noun in front of it. It tells the reader which one you mean, and the sentence changes meaning without it. A clause starting with which adds a comment about a noun the reader can already identify, and the sentence still works if you remove the clause.
Compare these two sentences. In the first, the clause is essential: The report that landed on my desk this morning needs a rewrite. Without the clause, you would not know which report. In the second, the clause is extra: The quarterly report, which landed on my desk this morning, needs a rewrite. The reader already knows which report; the arrival time is bonus detail, so it sits between commas.
The Comma Test
When you are unsure, try the comma test. Read the sentence without the clause. If the sentence still identifies the right thing, the clause is non-essential: use which and keep the commas. If removing the clause leaves the reader guessing, the clause is essential: use that and drop the commas.
Watch what happens here: The car that has the flat tire is in the garage. Remove the clause and you get The car is in the garage, which no longer says which car. Essential, so that is right. Now try: My car, which has a flat tire, is in the garage. Remove the clause and the sentence still works, because my car already identifies the vehicle. Extra, so which with commas is right.
Common Mistakes to Watch For
- Using which without commas for an essential clause. This is the most frequent error, and it makes the sentence read as if the clause were an aside.
- Using that with commas for an extra clause. Commas plus that is a mismatch in American style; pick one pattern or the other.
- Assuming which is always more formal. Formality does not decide this; essential versus extra does.
- Rewriting a correct that clause into a which clause just because it sounds softer. If the clause restricts the noun, keep that.
Does British English Differ?
Yes, somewhat. British writers use which for essential clauses more often than American writers do, and many British style guides accept it. American style guides, including the major news and academic manuals, keep the distinction strict. If you write for an American audience, follow the rule above. If you write for a British audience, either pattern is usually accepted, but staying consistent within a document matters more than which option you pick.
What About People?
For people, who usually replaces both words: the editor who approved the draft, not the editor that approved the draft. Using that for a person is not wrong, but who reads more naturally and is the safer choice in formal writing. The same essential-versus-extra logic applies: Ms. Patel, who approved the draft, is on leave uses commas because the clause is extra.
A Quick Decision Checklist
- Find the clause and the noun it describes.
- Delete the clause and read the sentence again.
- If the meaning breaks or the noun becomes unclear, use that with no commas.
- If the sentence still works, use which with commas.
- Read the result aloud. A which-clause should sound like a brief aside, not like a definition.
Once the comma test becomes habit, which vs. that stops being a guessing game. Check whether the clause restricts or merely comments, and the right word usually picks itself.