Defining clauses are clauses that begin with a relative pronoun and are essential to the meaning of a sentence. You might also refer to them as restrictive relative clauses, restrictive clauses or defining/restrictive adjective clauses. Essentially, defining clauses serve as an adjective that provides additional information or details about the word, noun or phrase they modify. Keep in mind that defining clauses are clauses because they contain both a subject and a predicate, which tells something about the subject and contains a verb; without a verb or without a subject, they are phrases. Likewise, they are also dependent clauses because they cannot stand alone as a sentence.
Relative pronouns are used to introduce relative clauses; whether they are defining or non-defining depends on the context of the sentence in which they appear. “Who,” “whom,” “that,” “which,” “whoever,” “whomever” and “whichever” are relative pronouns. In addition, “what,” “where” and “when” sometimes function as one as well. When used in a clause that serves as an adjective (relative clause), pronouns are sometimes subjects, sometimes objects or, in the case of whom, sometimes a possessive pronoun.
A defining clause provides meaning to a noun that is ambiguous, where the reference to which specific person or thing you are referring is not always clear. It does this through functioning as an adjective that defines more about the subject to identify a particular one. Consider the following examples:
Example 1: The cat that broke its leg was no longer in a cast.
If you ask, “which cat?” The answer is the one that broke its leg – making the relative clause “that broke its leg” essential to the meaning of the sentence; otherwise, “the cat” could refer to any of the many, many cats in any geographic location.
Example 2: The teacher has two boys in class who are rather rambunctious.
If you ask, “which boys?” The answer is the ones who are rambunctious – making the relative clause “who are rather rambunctious” essential to the meaning of the sentence because it shows there are more than two boys in the teacher’s class, but only two of them are rambunctious; otherwise, “two boys” implies that the teacher only has two boys in the class total, who are also rambunctious.
A non-defining clause, then, is a relative clause that is not essential to the meaning of the sentence. When a noun is more specific, the relative clause only provides additional information, not information that is essential to a sentence’s meaning or that gives an explanation of which one of many the subject is referring.
Example 3: Jonathan, who is the shortest boy in his class, is an avid action-movie watcher.
“Who is the shortest boy in his class” is a non-defining relative clause because you know that Jonathan is the person who likes action movies. The fact that he is the shortest boy in his class is just additional information. If “the boy” replaced “Jonathan” in the sentence, the relative clause turns into a defining one because it tells you which boy in a class likes action movies.
Example 4: Colleen invited friends over for dinner, which consisted of grilled shrimp, potatoes and asparagus.
“Which consisted of grilled shrimp, potatoes and asparagus” is a non-defining relative clause because you do not need to know what was for dinner for the sentence to retain its meaning. It is just additional information.
As a general rule, defining clauses are not offset with commas as they are essential and provide clarity, and non-defining ones are offset with commas because they are not essential for a sentence’s meaning. Consider the below examples:
Defining: The girl who loves music was happy to attend the concert.
Non-defining: Darcy, who loves music, was excited to attend the concert.
Note: The relative pronoun “that” is always used in a defining clause and is never set off with commas. However, the relative pronoun “which” is almost always used in non-defining clauses with offsetting commas.
A relative pronoun that serves as the object of the clause is sometimes omitted from a defining clause. Consider the below example:
The hat that the boy wanted the most sported his favorite sports team’s logo.
The hat the boy wanted the most sported his favorite sports team’s logo.