Which versus that: identify the group.
A relative clause can identify a group or add information about an established group. The difference changes the claim. My preferred pattern makes that difference visible through wording and punctuation.
By Adam Frick · First draftThe papers that need revision are on the desk.
The papers, which need revision, are on the desk.
A reader needs to know how far a claim extends. A relative clause can narrow the subject to a particular group, or supply information about a group already identified. That distinction matters more than a preference for one pleasant-sounding word. I recommend a consistent pattern for international writing: use “that” for defining clauses about things and use “which” with appropriate commas for supplementary clauses.
Look at a published definition.
Dual-use items are goods, software and technology which can be used for both civil and military applications.
Dual-use items are goods, software and technology that can be used for both civil and military applications.
The original uses “which” to help define the relevant category. My revision changes only the relative pronoun. The defining clause still identifies the capability that makes the items dual-use. This example demonstrates a proposed consistency convention, not the correction of an ungrammatical British sentence.
Change the punctuation and change the claim.
The reports that contain personal data require restricted access.
The reports, which contain personal data, require restricted access.
The first sentence restricts the requirement to reports containing personal data. The second describes the reports under discussion as containing personal data and requiring restricted access. The comma pair gives the clause a supplementary role. Replacing a pronoun without checking the punctuation cannot reliably preserve that distinction.
A common shortcut asks whether the sentence survives after the clause is removed. That test checks grammatical completeness but does not settle the intended meaning. Removing “that contain personal data” leaves a complete sentence while broadening the requirement to the reports as a whole. Ask which reports the reader should include before deciding which words to remove.
Teach a convention without inventing a prohibition.
The British Council’s grammar guidance allows “which” as well as “that” in defining clauses about things. My narrower recommendation gives writers one repeatable way to mark the distinction. A reader should not need to guess whether a clause selects a group or comments on an established group. Consistency can support that aim without turning a permitted alternative into an error.
A preposition can require another construction, such as “the method by which we measured the sample”. Do not replace “which” mechanically in that phrase. My recommendation concerns the defining-versus-supplementary contrast in suitable relative clauses, not every appearance of either word. A rule that ignores the construction will create new problems.
Choose a sentence containing a relative clause about a thing. Identify the full group, then decide whether the clause selects part of that group. Compare the meaning with and without the clause. Only after that comparison should you choose the relative pronoun and punctuation.