An important comma rule

Comma rules are important; incorrect comma use is not only wrong, it can change the meaning of a sentence. Here is one of the most important comma rules that many writers don't understand.

• Use a comma to set off nonessential information and clauses. Generally, if you can take out the information set off by the commas and the sentence still makes sense, the information is nonessential and the commas are correct. If the sentence no longer makes sense when the information is removed, the commas should be deleted from the sentence.
• This sentence is wrong: "Those residents of San Francisco, who do not hold secure well-paying jobs, must resent the common portrayal of the city as a land of opportunity." The information inside the two commas is essential and should not be set off by commas.
• This version is correct: "Those residents of San Francisco who do not hold secure well-paying jobs must resent the common portrayal of the city as a land of opportunity." No commas are necessary since we are qualifying which residents we talking about.