Relative clauses are dependent clauses introduced by a Relative Pronoun (that, which, whichever, who, whoever, whom, whomever, whose, and of which). Relative clauses can be either restrictive or nonrestrictive. Review the section on Comma Usage for additional help in determining whether relative clauses are restrictive or nonrestrictive (parenthetical or not) and whether commas should be used to set them off from the rest of the sentence. In a relative clause, the relative pronoun is the subject of the verb (remember that all clauses contain a subject-verb relationship) and refers to (relates to) something preceding the clause.

• Giuseppe said that the plantar wart, which had been bothering him for years, had to be removed.

(In this sentence, the clause in this color is a restrictive [essential] clause [a noun clause — see below] and will not be set off by a comma; the underlined relative clause [modifying "wart"] is nonrestrictive [nonessential — it can be removed from the sentence without changing the meaning of the sentence] and is set off by commas.)

Some relative clauses will refer to more than a single word in the preceding text; they can modify an entire clause or even a series of clauses.

• Charlie didn't get the job in administration, which really surprised his friends.
  
  
  
• Charlie didn't get the job in administration, and he didn't even apply for the Dean's position, which really surprised his friends.

A relative clause that refers to or modifies entire clauses in this manner is called a sentential clause. Sometimes the "which" of a sentential clause will get tucked into the clause as the determiner of a noun:

• Charlie might very well take a job as headmaster, in which case the school might as well close down.