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).

Note:

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.