“that” vs. “which”
Use "that" with no commas for information the sentence needs to identify its noun; use "which" with commas for extra information the sentence could lose without changing its core meaning.
that
"That" introduces a restrictive (defining) clause — one that narrows down which specific thing you're talking about, and that the sentence can't lose without losing its meaning ("the book that I borrowed is overdue," meaning specifically the borrowed one, not any book). "That" is one of English's oldest function words, tracing back to Old English "þæt," originally the neuter form of the demonstrative pronoun system that also gave us "this" and "those," and it took on its clause-introducing job very early in the language's history.
which
"Which" introduces a nonrestrictive (nondefining) clause — extra, removable information set off by commas that adds detail without narrowing down the noun ("the book, which I borrowed last week, is overdue," where the borrowing detail is a side note about a book already identified). "Which" comes from Old English "hwilc," a question word originally meaning "of what kind," and only later settled into its modern job of introducing supplementary relative clauses in standard written English.
The real distinction
The traditional American style-guide rule, popularized by Strunk and White's "Elements of Style," is a comma test: no commas plus "that" means the clause is essential to identifying the noun; commas plus "which" means the clause is just added color you could delete without losing the sentence's core meaning. "The cars that were recalled are in the lot" identifies a specific subset of cars (the recalled ones); "the cars, which were recalled, are in the lot" tells you something extra about cars already identified elsewhere. British English is considerably more relaxed about this distinction, and even careful British writers routinely use "which" for restrictive clauses without commas, a usage American style guides treat as an error but that is entirely standard on the other side of the Atlantic. This is one of the relatively few grammar rules that's genuinely a style convention rather than a hard grammatical law — sentences using "which" restrictively are perfectly understandable and grammatical, just not what most American editors will accept in formal prose. Because the difference often hinges entirely on the presence or absence of commas around the clause, proofreading for this pair means checking punctuation as much as word choice; a misplaced comma can accidentally turn an essential detail into a throwaway aside or vice versa. Legal and technical writing is particularly strict about this distinction, since a contract clause introduced with "which" instead of "that" (or vice versa) can genuinely change what a sentence is legally claiming to restrict.
Examples with “that”
- The laptop that crashed yesterday is back at the shop.
- I finally read the novel that you recommended.
- The recipe that uses buttermilk turned out best.
- She kept the one photo that survived the flood.
- The bridge that collapsed last year has been rebuilt.
- He wore the jacket that his father once owned.
- The app that saves the most battery won.
- We chose the hotel that had the best reviews.
Examples with “which”
- My laptop, which is three years old, finally crashed.
- The novel, which won several awards, is being adapted for film.
- Her recipe, which uses buttermilk, always turns out well.
- The bridge, which was built in 1932, needed repairs.
- His jacket, which once belonged to his father, is his favorite.
- The app, which saves battery automatically, updated last night.
- Our hotel, which overlooks the harbor, was fully booked.
- The report, which took months to finish, was well received.
Memory trick
Picture commas as parentheses in disguise: if you could swap the commas for actual parentheses and the sentence would still make full sense with the clause deleted, you need "which"; if deleting the clause would leave the sentence unable to say which specific thing it means, you need "that." One more useful check: read the sentence with the clause removed entirely — if it's still just as specific and complete, "which" (with commas) was correct; if the sentence suddenly becomes vague about which noun it's referring to, "that" (no commas) is what the sentence actually needs.