Tuesday, July 24, 2012

The trouble with "that" and "which": Essential clause vs. nonessential clause




When writers confuse that and which, essential/nonessential clauses are usually involved.

Essential and nonessential clauses give additional information about a word or phrase. The difference between them is an essential clause cannot be removed without changing the intended meaning of the sentence, thus making it necessary – or essential - to the sentence.

Here are the rules to remember:




ESSENTIAL CLAUSE =                                      NONESSENTIAL CLAUSE =

Starts with that                                              Starts with which

Not set off by commas                                    Set off by commas

Necessary for intended                                   Not necessary for intended

    meaning of sentence                                       meaning of sentence



Let’s try some examples:


Pit bulls that are too aggressive should be illegal to own.

Pit bulls, which are too aggressive, should be illegal to own.

            In the first sentence, the writer is saying a specific set of dogs - pit bulls “that are too aggressive” – should be illegal. If the “that are too aggressive” clause is removed, the intended meaning of the sentence is changed to target all pit bulls. So, the phrase “that are too aggressive” is essential to the meaning of the sentence.

            In the second sentence, the writer is saying all pit bulls should be illegal. Removing the “which are too aggressive” clause doesn’t change the meaning of the sentence. So, the phrase “which are too aggressive” is not essential to the sentence.


The key that starts the car has a square head.

The key, which starts the car, has a square head.

         In the first sentence, the writer is saying a specific key – the one used to start the car – has a square head. If the essential clause “that starts the car” is removed, then the intended meaning is changed to refer to a key in general.

            In the second sentence, the writer is referring to a general key, which just happens to start the car.


Incorrect: The loaf of bread, that is a day old, can be fed to the birds.

Incorrect: The loaf of bread which is a day old can be fed to the birds.

Correct: The loaf of bread that is a day old can be fed to the birds.

Correct: The loaf of bread, which is a day old, can be fed to the birds.

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