What Is a Relative Clause?
When we think of adjectives, we usually envisage a single word used before a noun to modify its meanings (e.g., tall man, smelly dog, argumentative employee). However, adjectives also come in the form of relative clauses (also called adjective clauses).
A relative clause comes after the noun it modifies and is made up of several words, which (like all clauses) include a subject and a verb.
Examples of Relative Clauses
- The windows that you installed last year have warped.
- Kindness is the language which the deaf can hear and the blind can see . (Writer Mark Twain)
- The follies which a man regrets most in his life are those which he didn't commit when he had the opportunity . (US journalist Helen Rowland) (Even though this is a quotation by an American journalist, nowadays, most writers in the US would use "that" instead of "which." There's more on this below.)
The Traits of a Relative Clause
A relative clause has the following three traits:
A relative clause is headed by a relative pronoun ("who," "whom," "whose," "that," or "which") or a relative adverb ("when," "where," or "why"), which links it to the noun it is modifying.
Quite often, the relative pronoun can be omitted. However, with a relative clause, it is always possible to put one in. (There is more on this below.)
A relative clause has a subject and a verb. (These are what make it a clause.)
A relative clause tells us something about the noun. (This is why it is a type of adjective.)
More about the Three Traits of a Relative Clause
Look at the three traits in this example: 
Often, the relative pronoun is the subject of the clause. Look at the three traits in this example: 
Dropping the Relative Pronoun
It is common for the relative pronoun to be omitted. For example:
- The windows
that you installed last year have warped.
- The film
which you recommended scared the kids half to death.
- The follies
which a man regrets most in his life are those which he didn't commit when he had the opportunity . (Helen Rowland, 1876-1950)
This is not always possible though:
- Bore: a person
who talks when you wish him to listen .
When the relative clause starts with a relative adverb ("when," "where" or "why"), the relative adverb cannot be omitted.
- There comes a time when money doesn't matter . (Boxer Floyd Mayweather) (You can often omit a relative pronoun, but you can't omit a relative adverb. So, you can't omit "when" in this example.)
Why Relative Clauses Are Important
Here are two common questions related to relative clauses.
(Question 1) Should I use a comma before "which?
This is the most common question related to relative clauses. The answer is sometimes yes and sometimes no. The answer applies to all relative clauses, not just those that start with "which."
- Do not put commas around your clause if it is essential; i.e., it is required to identify its noun. (This is called a restrictive clause.)
- Do use commas if your clause is just additional information. (This is called a non-restrictive clause.)
A Good Tip
If you'd happily put parentheses (brackets) around your clause or even delete it, then use commas.
Here is an example of a non-restrictive clause: 
- My brother, who claimed to have a limp , sprinted after the bus. (This clause is not required to identify "My brother." It is just additional information.)
- My brother (who claimed to have a limp) sprinted after the bus. (As it's just additional information, you can put it in brackets.)
- My brother sprinted after the bus. (As it's just additional information, you can even delete it.)
Compare this to a restrictive clause: 
- A burglar who fell through a garage roof is suing the house owner. (This clause is required to identify "a burglar." Without it, we don't know which burglar we're talking about.)
- The tramp (who claimed to have a limp) sprinted after the bus. (This sentence is only appropriate if we know which tramp we're talking about.)
- The tramp sprinted after the bus. (This sentence is only appropriate if we know which tramp we're talking about.)
(Question 2) What's the difference between "that" and "which"?
"Which" and "that" are interchangeable, provided we're talking about "which" without a comma.
When "which" starts a restrictive clause (i.e., a clause not offset with commas), you can replace it with "that." In fact, Americans will insist you use "that" instead of "which" for a restrictive clause.
- Mark's dog which ate the chicken is looking guilty. (but in America or at least widely disliked) (Americans baulk at "which" without a comma. They insist on "that.")
- Mark's dog that ate the chicken is looking guilty. (This version is acceptable for all. It will stop you getting hate mail from Americans.)
For many, even those following UK conventions, "that" feels more natural with a restrictive clause. This feeling is something we can use. If all this talk of restrictive and non-restrictive clauses is confusing, replace your "which" with "that." If your sentence still sounds good, then you almost certainly want "which" without a comma. This trick works because "that" can only be used with a restrictive clause, and – whether you consciously know it or not – some language-processing area of your brain does.
- The burglar who is suing the homeowner was booed in court.
- The burglar that is suing the homeowner was booed in court. (Substituting "who" for "that" is a good way to test whether an adjective clause needs commas or not, but some of your readers might not like "that" being used for a person – even a burglar. So, if your clause starting "who" sounds okay with "that," then revert to "who" without commas.)
Key Points
- If you'd happily delete your clause, then it's not a restrictive clause, and it should be offset with commas.
- Don't start a restrictive clause with "which" if you're American or writing to Americans. Use "that" instead.
- Use "who" not "that" for people. "Who" is a bit sharper.
This page was written by Craig Shrives.
