06 Mar A Grammar Blog–restrictive and non-restrictive clauses

The dog that mauled the child had to be put down.

The dog, which was the oldest of the animals, had to be put down.

In the first sentence, the clause that mauled the child is essential to the sentence because it’s pointing out a particular dog.  So within the context of that sentence, that mauled the child restricts, or narrows down, the meaning of “the dog”.  This is a restrictive clause.

In the second sentence, the clause which was the oldest of the animals isn’t essential to the meaning of the sentence.  It simply gives more information about the dog.  This is a non-restrictive clause.

Usually, non-restrictive clauses are properly separated by commas.