that / which
Restrictive versus nonrestrictive relative clauses.
Use that for a restrictive clause. Do not set the clause off with commas: the report that arrived this morning. Use which for a nonrestrictive clause. Set the clause off with commas: The report, which arrived this morning, is on your desk.
A restrictive clause identifies which noun is meant. A nonrestrictive clause adds information about a noun already identified.
That does not introduce a nonrestrictive clause.
Use which in a restrictive clause only after a preposition (the drawer in which the files are kept) or immediately after the pronoun that (that which was ordered).
See also restrictive / nonrestrictive clauses; comma with nonrestrictive clauses; where / that / in which in relative clauses; who / whom; who vs. that for people.
Examples
UseThe crate that blocked the loading dock is still there. (The clause identifies which crate is meant.)
UseThe crate, which blocked the loading dock, is still there. (The crate is already identified; the clause adds a fact.)
AvoidThe crate which blocked the loading dock is still there. (Which is the error: the clause identifies which crate is meant. Write The crate that blocked the loading dock is still there.)
AvoidThe proposal, that the committee approved on Tuesday, is ready for review. (That is the error: the clause adds a fact about a proposal already identified. Write The proposal, which the committee approved on Tuesday, is ready for review.)
Restrictive which is standard in British English and appears often in American writing. Reserving that for restrictive clauses became the American convention in the twentieth century. Many earlier writers used restrictive which.