Types of relative clauses
To understand the distinctions between that and which it is necessary to understand defining (restrictive) and non-defining (non-restrictive) clauses.
Learning these distinctions is one technical aspect of grammar that every user of English should understand, because it is at the root of an assortment of grammatical errors.
1. Defining clauses (Restrictive clauses)
A defining (restrictive) clause is one that is essential to the sense of the sentence.
My house that has a blue door needs painting.
Here the blue door is a defining characteristic, it helps to distinguish that house from my other houses.
Here the blue door is a defining characteristic, it helps to distinguish that house from my other houses.
Defining clauses or phrases are not separated off with commas. A restrictive clause or phrase is essential to the meaning of the sentence; it defines the word it modifies by ›restricting‹ its meaning. Eliminating a restrictive element from a sentence changes its meaning dramatically.
We can use relative clauses to join two English sentences, or to give more information about something.
I bought a new car. It is very fast.
→ I bought a new car that is very fast.
→ I bought a new car that is very fast.
She lives in New York. She likes living in New York.
→ She lives in New York, which she likes.
→ She lives in New York, which she likes.
Defining and Non-defining
A defining relative clause tells which noun we are talking about:
- I like the woman who lives next door.
(If I don't say 'who lives next door', then we don't know which woman I mean).
2. Non-defining clauses (Non-restrictive clauses)
A non-defining (non-restrictive) clause is one that can be regarded as parenthetical:
My house, which has a blue door, needs painting.
The italicized words are effectively an aside and could be deleted. The real point of the sentence is that the house needs painting; the blue door is incidental.
Use commas to set off non-defining elements, which contribute to, but do not determine, the meaning of the sentence. These elements may be clauses (groups of words that contain a subject and a verb) or phrases (groups of words that do not contain both a subject and a verb).
Tip:
Note that defining (restrictive) and non-defining (non-restrictive) clauses must be introduced by the appropriate relative pronoun (who, which, that, whose).
Defining (restrictive) clauses should NEVER be set off with commas and non-defining (non-restrictive) clauses ALWAYS should.
Additional information
On that much the authorities are agreed. Where divergence creeps in is on the question of how strictly the distinctions should be observed.
- In defining (restrictive) which is used in British English, that in American English.
I like the book which I bought yesterday.
I like the book that I bought yesterday. - In non-defining (non-restrictive) which is used both in British and American English.
The smartphone, which he bought yesterday, is 5 months old.
The smartphone, that he bought yesterday, is 5 months old.
