Conventions of Composition Rule 9

Rule: An essential or restrictive (necessary to the meaning of the sentence, explaining which particular person or thing) adjectival clause doesn't need to be set off with commas. An inessential or non-restrictive (not necessary to the meaning of the sentence, provides added information about a known noun or pronoun) adjective clause should be separated from the rest of the sentence with commas.

Examples:

Correct: The man, who is standing by the door, is my uncle. (There is only one man in the room.)

Also correct: The man who is standing by the door is my uncle. (There are several men in the room, only one of whom is near the door.)

Correct: I want an overcoat that will keep me warm. (I don't want any other kind of overcoat.)

Correct: The year when he died is unknown. (Without "when he died," the sentence doesn't make sense.)

Correct: My father, who was penniless at twenty, is now a millionaire. (I have only one father, so "who was penniless at twenty" doesn't clarify which father.)

Practice deciding which sentences need commas, where they go, and what each sentence means depending on where or not you add commas:

1. Chicago where she was born has the best restaurants.
2. The road with the six purple houses leads directly to the asylum.
3. My uncle who wears a beret teaches Spanish.
4. In the waiting room where I found a whole cheesecake I had a feast.

Resources for further explanation of using commas with intervening clauses:

Purdue OWL's Commas

dj05cc's (on Youtube) Using Commas in Adjective Clauses