There are two types of relative clauses—defining and nondefining. To review, relative clauses can contain a subject, verb, and a relative pronoun, though not all are needed. The relative pronouns are who, whose, when, where, which, and that. Relative clauses are sometimes called adjective or adjectival clauses because they describe nouns like adjectives do. Defining clauses give essential information about the main noun. Often, they answer the question: “Which one?” If you haven’t already guessed, nondefining clauses do not define nouns. But what do they do?
Nondefining relative clauses provide supplementary information. However, the information is not key to the meaning of the sentence. In fact, the sentence would still make sense if you removed the nondefining clause. Look at these example sentences with and without nondefining clauses.
The country of Costa Rica has extensive measures in place to protect endangered rainforest animals. (This sentence does not have a relative clause.)
The country of Costa Rica, where extinct species such as the golden toad once thrived, has extensive measures in place to protect endangered rainforest animals. (The nondefining clause “where extinct species such as the golden toad once thrived” tells us more about Costa Rica, but nothing that is essential to understanding of the sentence.)
Besides the type of information added (essential vs. nonessential), the use of commas is different between the two types of clauses. Commas set off nondefining clauses from the rest of the sentence. Defining clauses do not need commas.
Was your guess correct? Celebrate by adding some nondefining clauses to your writing. They add supplementary information. They are not essential, but they can be quite interesting.