1. Ask two questions instead of mechanically choosing a word
First establish what the relative clause does. A defining or restrictive clause answers which one? or limits a class of things. A non-defining or non-restrictive clause adds a separate statement about an item or group already identified in the context. Commas express this difference in writing.
Do not check only whether removing the clause leaves a grammatical sentence. It often does, but the set of things being discussed changes. In an instruction to open documents containing a signature, removing the signature requirement removes the selection criterion, even though the remaining sentence is complete.
Open the documents that contain a signature.
Open the documents that contain a signature.
There are different documents; a signature identifies the required group. Do not set off the defining clause with commas.
Open the documents, which contain a signature.
Open the documents, which contain a signature.
The listener already knows which documents are meant. The which-clause adds a property of that identified group rather than selecting it.
2. Which can also define
In an ordinary defining relative clause about things, with no preposition directly before the relative word, both that and which are possible. Changing one to the other does not itself turn a defining clause into a non-defining one. Which alone does not automatically require commas.
Some house styles require that in defining clauses and reserve which for non-defining clauses. Follow such a requirement when explicitly specified for a text. It is a style convention, not a general ban on defining which, and not a universal rule of American English.
In modern neutral written English, a non-defining clause about things uses which, not that. This is where the choice between the two words is grammatically restricted.
identifying modifier: noun + that / which + relative clause; additional information: noun, which + relative clause
- noun —
- A word naming a person, thing or idea: student, book, happiness.
- clause —
- A construction with a verb, often with its own subject: when I arrived. A non-finite clause may leave the subject unspoken: to get home.
The filter which removes dust needs replacing.
The filter which removes dust needs replacing.
Which removes dust identifies the filter; that removes dust is equally correct.
The filter, which removes dust, needs replacing.
The filter, which removes dust, needs replacing.
The filter is identified independently of this additional property. You cannot replace which with that while retaining the commas and construction.
3. Commas mark the boundaries of additional information
If a non-defining clause appears within a sentence, use two commas: one before which and one after the whole relative clause. The second returns the reader to the main clause. Do not separate the relative word from its predicate with a comma.
If the additional information ends the sentence, use one comma before it and the usual full stop at the end. Do not add a second comma before the full stop. These exercises practise comma punctuation specifically, although other editing contexts sometimes allow brackets or dashes for parenthetical information.
Do not set off a defining relative clause with commas. This does not mean the entire sentence must contain no commas: a comma may belong to an introductory phrase or another construction.
The south entrance, which faces the river, is closed.
The south entrance, which faces the river, is closed.
Which faces the river is supplementary information. The closing comma before is closed returns us to the main message.
We used the south entrance, which faces the river.
We used the south entrance, which faces the river.
The extra information ends the sentence, so only a full stop follows river.
After lunch, inspect the machine that overheated.
After lunch, inspect the machine that overheated.
The comma after lunch belongs to the introductory adverbial, not to defining that overheated.
4. Identifiable in context does not mean unique in the world
A non-defining clause signals that the reader can identify the referent without that clause. The item may have been mentioned, pointed to, or made clear by the situation. My car, which… does not prove that the owner has only one car.
With a plural noun, an entire set may already be identified by an earlier statement. The additional property applies to that set, not to every such item in the world.
A defining clause selects the group about which a statement is made, but usually does not say what happened to the rest. The files which were corrupted were restored asserts that the corrupted files were restored. It does not assert that no uncorrupted files were restored, and it gives no file count.
My car, which is parked outside, needs a new tyre.
My car, which is parked outside, needs a new tyre.
The conversation concerns an already identified car. The sentence does not establish whether I own other cars.
The samples that passed the test were stored.
The samples that passed the test were stored.
The sentence reports what happened to the samples that passed. The other samples' fate requires a separate statement.
5. Omitting the pronoun: role matters more than length
In a defining clause, the relative word can be omitted when it is the object: the report (that / which) I read. The subject I is already present, and the omitted word corresponds to the object of reading. The zero-pronoun version needs no commas.
If that or which is the relative clause's subject, simple omission is not possible: the report that arrived, not the report arrived in this relative construction. This does not ban the separate sentence The report arrived; it is a requirement for the relative-clause link.
Do not omit the pronoun in a non-defining clause, even when it is the object. Do not duplicate it with a separate it or they in the same grammatical role: which I read already contains the object, while which arrived contains the subject.
The guide I borrowed is excellent.
The guide I borrowed is excellent.
A defining clause with the relative word as the object of borrowed. The fuller the guide that I borrowed and the guide which I borrowed are also possible.
The guide, which I borrowed yesterday, is excellent.
The guide, which I borrowed yesterday, is excellent.
The guide is already identified. Which is required in the additional clause even though I is the subject.
The guide which arrived yesterday is excellent.
The guide which arrived yesterday is excellent.
Which is the subject of arrived. It can be replaced by that but cannot simply be omitted.
6. Boundaries: prepositions and comments on an event
When a preposition directly precedes the relative word referring to a thing, use which: on which, with which. On that does not fit this relative construction. This is a restriction on the preposition's position, not evidence that every which-clause is supplementary. Full stylistic restructuring with prepositions is practised separately.
Which can also comment on the entire preceding fact: in The screen froze, which delayed the presentation, the freezing caused the delay, not the screen itself. This is also a supplementary relative comment with a comma, but its reference is an event rather than a noun. Non-defining and referring to the whole fact are not mutually exclusive categories.
If which could theoretically refer to either an item or an event, do not guess the intended meaning. Add context or rephrase. The main skill here is making the reference and message boundaries clear, not assigning an arbitrary single label to an ambiguous sentence.
The platform on which the equipment stands is stable.
The platform on which the equipment stands is stable.
The clause identifies the platform without commas, but directly after on we use which.
The screen froze, which delayed the presentation.
The screen froze, which delayed the presentation.
Which refers to the fact that the screen froze. Replacing it with that after this comma would not create a correct relative clause.
7. Edit the intended meaning, not just the symbols
Before correcting a sentence, identify the meaning to preserve. In The boxes, that arrived today, are upstairs, you can replace that with which to retain an additional statement about already identified boxes. Or you can remove both commas, selecting the boxes by their arrival time. Both results are grammatical, but they are not the same editorial change.
In a work email or instruction, first identify the item clearly enough. Then decide whether the following property selects it or merely adds information. For a defining clause, choose an appropriate that, which, or omitted object pronoun. For a non-defining clause, use which and mark the full boundaries of the addition.
When checking your work, read only the main clause, then restore the relative clause. Has the intended set of items changed? Have you added an inference about the number or the fate of the remaining items? Model open answers show one possible solution; other natural wording and structures are acceptable when they meet the stated goal.
The boxes that arrived today are upstairs.
The boxes that arrived today are upstairs.
The intended meaning distinguishes today's delivery from other boxes. Which is also possible instead of that.
The boxes, which arrived today, are upstairs.
The boxes, which arrived today, are upstairs.
The boxes are already identified; their arrival time is additional information. The commas change the property's role.
Common mistakes
Incorrect:The old bridge, that crosses the canal, is closed.
Correct:The old bridge, which crosses the canal, is closed.
In this comma-marked non-defining construction, that cannot introduce the extra information about the bridge. Replacing it with which preserves the intended meaning and punctuation.
Incorrect:The printer which it failed yesterday is offline.
Correct:The printer which failed yesterday is offline.
Which is already the subject of failed; it incorrectly duplicates that subject. Remove only it.
Incorrect:The folder, I shared yesterday, contains the receipts.
Correct:The folder, which I shared yesterday, contains the receipts.
A non-defining clause cannot omit its object relative pronoun. Restoring which preserves the supplementary nature of the information.
Incorrect:The cable with that we connected the screen is damaged.
Correct:The cable with which we connected the screen is damaged.
Directly after with in a relative clause about a thing, use which, not that. The absence of commas does not make with that acceptable here.