Which sets off unessential information and, therefore, requires a comma. Commas separate introductory phrases from the sentence's subject. Known to strike terror in timid hearts, the comma is a simple friend. Commas separate big numbers . . . I ate 1,237 peas at one sitting.
Commas eliminate confusion with repeating words.
What the matter is, is hard to know. Commas hate interrupting one subject that works with only two verbs. My boss drew up plans and left on a vacation. One writing problem is the reference to the antecedent . . . John told Bill John's car was ugly .