Another use of the comma is to enclose parenthetic material. Words that can safely be left out without affecting the meaning of the sentence are considered parenthetical. (My son, you will be pleased to know, is now living in London.) or (My son, Richard, is now living in London.) A name or a title in direct address is parenthetic and is always set off by commas. (Richard, when are you coming home for a visit?)
No comma should separate a noun from a restrictive term of identification (Billy the Kid, William the Conqueror).
Along with nonrestrictive clauses, clauses introduced by conjunctions indicating time and place are also parenthetic. Consequently, these need a comma. A nonrestrictive clause is one that does not identify nor define the antecedent noun. (In 1941, when Pearl Harbor was bombed, I had not been born.)
The word “which” is often used to set off a nonrestrictive clause. (My dog, which is black and furry, is named Kavic.) It does not matter that my dog is black and furry; that description is irrelevant. Go on a “which hunt” to see how many “whiches” should be turned into “thats.”
Restrictive clauses are not parenthetic and are not set off by commas. They are necessary to the meaning of the sentence. (The group who sat in the back cheered loudest of all.) The word “that” is often used to introduce a restrictive clause. (The houses that were located in the woods were in danger of catching on fire.) All the houses were not in danger of the fire, only the ones located in the woods. It changes the meaning of the sentence to eliminate “in the woods.” The word “that” can often be omitted without hurting the meaning of the sentence as is possible here. (The houses located in the woods were in danger of catching on fire.)