Relative clauses: that, which, who and whose
A relative clause adds information about a noun or a larger statement. Its position, pronoun and punctuation show what it refers to and whether it identifies a subset or adds a comment about something already identified.
Forming a relative clause
Two sentences, “Sarah speaks 5 languages” and “Sarah works as a translator”, can be combined by replacing one occurrence of “Sarah” with “who”: “Sarah, who speaks 5 languages, works as a translator” or “Sarah, who works as a translator, speaks 5 languages”. The relative clause is dependent and acts adjectivally, modifying a noun in another clause. Combining short sentences this way can make them smoother; reversing the process can break a complicated sentence into shorter, simpler ones.
Relative pronouns and adverbs
Relative pronouns differ in their referents and grammatical roles.
| Pronoun | Referent and role |
|---|---|
| That | People, animals or things, as subject or object. |
| Who | One or more people, as subject or predicate complement. |
| Whom | People, as direct or indirect object or object of a preposition. |
| Which | Animals or things, as subject or object; it can also modify a noun as a relative adjective or follow a preposition, as in “for which” or “at which”. |
| Whose | Possession; sometimes classified as a relative adjective. |
“Whose” can have an inanimate referent: “Nothing is more powerful than an idea whose time has come.” Relative adverbs “where”, “when” and “why” can also introduce relative clauses.
Referents and position
A bound relative clause modifies a particular noun: in “They killed the goose that laid the golden eggs”, “that” is the clause's subject and links it to “goose”, the main clause's direct object. In a sentence diagram, a dotted diagonal line from “goose” to “that” shows the pronoun's linking role and the clause's adjectival role.
A relative clause can instead refer to a whole sentence or more, using “which”: “I don’t have to work tomorrow, which is great because the weather forecast is good.”
“A clause is a string of words that consists of a subject and a predicate” risks attaching “that” to “words”; “A clause is a word string that consists of a subject and a predicate” puts the clause after its intended noun.
A coordinating conjunction joins two relative clauses modifying the same noun. Repeating the relative pronoun can make the sentence easier to parse, although it can be omitted to create a compound predicate: “Testosterone is a hormone that is made mainly in the testes but that is also made by the adrenal glands.”
Defining and commenting clauses
A restrictive, defining or essential clause identifies a subset of what the noun names; a nonrestrictive, commenting or nonessential clause adds information without changing that scope.
“The vaccine covers the pneumococcal strains that cause invasive disease” concerns the invasive strains and says nothing about coverage of other strains. Adding “only” expressly excludes other strains, while adding “all” stresses complete coverage of the specified strains. By contrast, “The pancreas secretes insulin, which plays a key role in controlling blood sugar” adds a comment about insulin without restricting it to a subset.
“The houses that have a black roof are blue” describes only black-roofed houses and leaves other houses unaddressed; “The houses, which have a black roof, are blue” says that all the houses are blue and have black roofs.
The punctuation can carry the distinction with several relative pronouns.
| Defining | Commenting |
|---|---|
| “In the experiment which lasted two hours, the heat from the furnace was…” identifies which experiment. | “In the experiment, which lasted two hours, the heat from the furnace was…” comments on an identified experiment. |
| “Colour-coding highlights the discontinuous contours for the user who can use the editing tools to create continuous contours.” | “Colour-coding highlights the discontinuous contours for the user, who can use the editing tools to create continuous contours.” |
| “The medical information world was once dominated by librarians whose primary concern was the management of book collections.” | “The medical information world was once dominated by librarians, whose primary concern was the management of book collections.” |
The same distinction applies to samples stored at approximately −20 °C: “Samples that were not processed immediately were stored with residual carcasses at approximately −20 °C” identifies a subset; “Samples, which were not processed immediately, were stored with residual carcasses at approximately −20 °C” adds information about the samples collectively. The defining version can also use “Samples which were not processed immediately…” without a comma.
Pronoun choice and punctuation
Restrictive and nonrestrictive clauses allow different relative pronouns.
| Clause | Human referent | Nonhuman referent | Possessive |
|---|---|---|---|
| Restrictive | Who, whom, that | Which, that | Whose |
| Nonrestrictive, set off by punctuation | Who, whom | Which | Whose |
“Which” can introduce a defining clause if it is not set off by punctuation. If replacing “which” with “that” leaves the intended message unchanged, the clause is defining.
Ambiguous references
“The cells sedimented to the bottom of the tube which was associated with a change of colour” is unclear about whether “which” refers to the tube's bottom or to sedimentation. Where doubt remains, the sentence should be rewritten completely: “Cells sedimented to the bottom of the tube, resulting in a change of colour” or “Cell sedimentation to the bottom of the tube led to a change of colour”.
“Our laboratory, which has two dark rooms, is located in the city centre” locates an identified laboratory and comments on its rooms. “Our laboratory that has two dark rooms is located in the city centre” implies several laboratories and identifies one; replacing “that” with “which” does not justify adding a comma, which would change the meaning.
In “The focus is on drugs, which have the potential to partition (dissolve) in a lipid membrane but do not perturb membranes”, the comma presents the relative clause as a comment about drugs generally. The intended focus was a subset of drugs with those properties, requiring a defining rather than a commenting clause.
In a bone-marrow-donation text, “especially first-degree relatives who have a higher probability of being HLA-compatible will be tested” appeared to restrict testing to relatives meeting an additional condition, potentially discouraging people unsure whether they qualified. The intended priority for first-degree relatives was expressed by “especially first-degree relatives, who have a higher probability of being HLA-compatible, will be tested”.
Omitted relative pronouns
A relative pronoun can sometimes be omitted, as in “the man [whom] I love”. Omitting a relative pronoun together with its finite verb, such as “that is” or “that were”, may turn the clause into a participial phrase and reduce the word count.
Editing checks
A recommended sequence of checks addresses both meaning and grammar.
- Determine whether the relative pronoun introduces an adjectival clause or a noun clause; for an adjectival clause, identify the modified noun and try to place the clause directly after it.
- Set nonrestrictive clauses off with commas, parentheses or dashes, and leave restrictive clauses unseparated. If the intended meaning is uncertain, query the author.
- Use “that” rather than “which” for restrictive clauses about nonhuman things to emphasise the restriction.
- Prefer “who” or “whom” for people to avoid dehumanising language.
- When unsure between “who” and “whom”, or “whoever” and “whomever”, diagram the sentence.