The relative pronouns (who/whoever/which/that) relate groups of words to nouns or other pronouns (The student who studies hardest usually does the best.). The word who connects or relates the subject, student, to the verb within the dependent clause (studies). Choosing correctly between which and that and between who and whom leads to what are probably the most Frequently Asked Questions about English grammar. Generally, we use "which" to introduce clauses that are parenthetical in nature (i.e., that can be removed from the sentence without changing the essential meaning of the sentence). For that reason, a "which clause" is often set off with a comma or a pair of commas. "That clauses," on the other hand, are usually deemed indispensable for the meaning of a sentence and are not set off with commas. The pronoun which refers to things; who (and its forms) refers to people; that usually refers to things, but it can also refer to people in a general kind of way. For help with who/whom refer to the section on Consistency. We also recommend that you take the quizzes on the use of who and whom at the end of that section.

The expanded form of the relative pronouns — whoever, whomever, whatever — are known as indefinite relative pronouns. A couple of sample sentences should suffice to demonstrate why they are called "indefinite":