Language
Defining and non-defining relative clauses
Tell a clause that says which one from a clause that only adds extra information, and punctuate each correctly.
A relative clause describes a noun. Two of them can look identical word for word and mean different things, and the only difference on the page is a pair of commas. That makes this one of the few places in English where punctuation carries the whole meaning.
Defining — no commas
The boy who scored the goal is my neighbor. Without the clause you do not know which boy, so the clause cannot be spared. Take the commas out and leave them out.
Non-defining — commas at both ends
Ravi, who scored the goal, is my neighbor. You already know who is meant; the clause is a spare remark. Commas at both ends — unless the clause ends the sentence, where the period closes it. Half-way commas anywhere else are a mistake.
The lift-it-out test
Read the sentence without the clause. If you still know exactly who or what is meant, the clause is non-defining and takes commas. If you are suddenly left asking "which one?", it is defining and takes none.
That, and which
That is only ever allowed in a defining clause — it can never sit between commas. In a defining clause both that and which are used, and which is perfectly ordinary in British writing, so do not treat it as an error. Non-defining clauses take who, whom, whose or which, never that.
When you may drop the pronoun
The book I borrowed was excellent. You can leave out who, whom, which or that when the clause has its own subject — here, I. You can never drop it from a non-defining clause.
Worked example
Punctuate: "The students who had finished early were sent to the library." and "My best friend who had finished early was sent to the library."
Lift the clause out of the first: "The students were sent to the library."
Which students? The sentence no longer says. So the clause is doing real work — it is defining.
Try it together
Let us punctuate one together.
The sentence is: My aunt ____ teaches history ____ has written a book. The speaker has only one aunt. Answer each step with a single word.
1.Lift the clause out: "My aunt has written a book." Do you still know exactly which aunt is meant? Write yes or no.
Have a go
There are thirty books on the shelf. Which version says that only some of them were damaged?
Ready to practice?
Eight questions on what you have just read. Nothing is timed, and you can play as many times as you like.