Defining and non-defining clauses

Commas are very important with a special group of clauses which are called essential and non-essential clauses, also called defining and non-defining clauses.

With essential and non-essential clauses it is important to use commas correctly because different uses can change the meaning of the sentence.

Essential clauses identify the person or thing that is being described. They are essential to understanding the sentence. They define the person or the thing. For example:

  • My sister who lives in France came to see us last month. (I have 2 sisters)
  • The woman who bought my house is an MP.

The clauses in bold are essential to the understanding of the sentences. If they are removed, the sentences lose their meaning. We won’t know which sister or which woman the speaker is referring to. These clauses often start with who, which or that. We do not use commas with clauses of this kind.

Non-essential clauses are not essential to the understanding of the sentence since they merely supply some additional information. They can be omitted from the sentence without changing the basic meaning. For example:

  • My MP, who lives just up the road, came round to see me yesterday.
  • The fisherman, who was wearing Wellington boots, came running up the road.

If we left out the underlined clauses it would not affect the meaning. Non-essential clauses like this often start with who or which but never that. When we use clauses of this type, we must use commas.

Try reading aloud the four examples given above. What do you notice? You might have noticed that where we place commas in the non-essential clauses, we also place a pause in our speech.

Here are some more examples of non-essential clauses in sentences; hence, we need commas.

  • Mr. Robinson, who is a retired lecturer, does voluntary work at the centre.
  • Tandi, who speaks Spanish, should apply for the job.
  • Malaria, which is widespread these days, is a dangerous disease.