A relative clause is part of a sentence that contains both a subject and a verb, but it is not a complete sentence in and of itself. This type of clause begins with a relative pronoun such as “who” or “which” and functions as an adjective in a sentence to provide more information about a noun or other object. Proper punctuation for a relative clause in a sentence depends upon whether it is a restrictive or nonrestrictive clause. It is restrictive if it helps identify the word that it modifies and is therefore essential, while nonrestrictive clauses are nonessential and simply provide additional information.
In general, a clause is part of a sentence, which can be a complete sentence on its own or requires more information to be a full sentence. A relative clause is dependent; this means that it is not a complete sentence on its own. There is a relative clause in the sentence “The boy who lives upstairs is my friend.” The section, “who lives upstairs,” is a clause that provides additional information about the subject of the sentence, referred to by the relative pronoun “who,” but is not a complete sentence.
Another example of a relative clause in a sentence is “The restaurant next door, where Bill had that great pizza, opens at noon.” In this example, the clause “where Bill had that great pizza” is relative and begins with the pronoun “where” that is followed by the subject of the clause, “Bill.” It provides additional information about the subject of the whole sentence, “The restaurant,” in much the same way that the clause in the previous example modified the subject, “The boy.” Both of these examples would become fragment sentences if used on their own, and they require additional information not only for grammatical completion but also for clarity of meaning.
Each example is also a different type of relative clause that requires a different type of punctuation. In the first example, the information it provides, “who lives upstairs,” is essential to the sentence as it identifies the subject. Without this the sentence would be “The boy is my friend,” which is vague, and so the clause is considered restrictive. No punctuation is required because it is essential.
In contrast to this, the relative clause in the second example is nonrestrictive and nonessential. Without “where Bill had that great pizza,” the sentence would be “The restaurant next door opens at noon.” In this example, the sentence still makes sense and the identity of the subject remains clear. The clause provides additional information, but does not identify the subject that it modifies, and so commas are used to separate it from the rest of the sentence.