Another way of making complex sentences is to use relative clauses. There are two types of relative clause: defining and non defining. This is an example of a defining relative clause – People who live in poor countries often have a lower life expectancy. In a defining relative clause, the information is required to make a complete sentence. People often have a lower life expectancy is not a complete sentence as we do not know who they are lower than. This is an example of a non-defining relative clause – Dogs, which make excellent pets, are often given as unwanted presents. In a non-defining relative clause, the information introduced with which is optional because the sentence Dogs are often given as unwanted presents is complete. Only non-defining relative clauses use commas to separate the additional information from the main sentence.