Relative clauses – Defining and non-defining – that, which
Types of relative clauses
To understand the differences between that and which it is necessary to understand what defining (restrictive) and non-defining (non-restrictive) clauses are.
1. Defining clauses (Restrictive clauses)
A defining (restrictive) clause is one that is essential to the sense of the sentence.
My car that has a flat tyre needs servicing.
Here the flat tyre is a defining characteristic, it helps to distinguish that car from my other cars.
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. Removing a restrictive element from a sentence changes its meaning dramatically.
2. Non-defining clauses (Non-restrictive clauses)
A non-defining (non-restrictive) clause is one that can be regarded as parenthetical:
My car, which has a flat tyre, needs servicing.
The words between the commas are effectively additional information and could be deleted. The real point of the sentence is that the car needs servicing; the flat tyre is an aside.
We use commas to highlight 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).
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 used with commas and non-defining (non-restrictive) clauses should ALWAYS.