A restrictive clause is a relative clause that introduces additional information that cannot be omitted. Restrictive clauses introduce necessary information about a noun, and they are not separated from the rest of the sentence by commas.
Compare nonrestrictive clauses.
For instance, in the following phrases:
- The computer that I bought last year has stopped working.
- The exam that she’ll take tomorrow will be her last one of the semester.
- The professor whose course I failed has retired.
- The boy who you saw in the cafeteria was my friend’s brother.
…the bold parts indicate the restrictive clause.
In restrictive clauses, the pronouns (in bold and italics) may be omitted, so that the previous phrases may become:
- The computer I bought last year has stopped working.
- The exam she’ll take tomorrow will be her last one of the semester.
- The professor course I failed has retired.
- The boy you saw in the cafeteria was my friend’s brother.