Last week, we looked at appositives and comma use. Basically, appositives rename or give additional information about nouns that immediately precede them. If that information selects a specific individual out of a larger group (as a proper name often does) or gives other essential information, we don't use commas. Not even a hint of type separates that information from its noun. (Think "My cousin Vinny" when one has about a dozen cousins, or "the 13th-century philosopher Thomas Aquinas" when there is a fleet of 13th c. philosophers under discussion.)

When the information is merely additional or renames a noun, we use commas to show that. "Thomas Aquinas, a 13th-century philosopher, became a saint." "Vinny, my cousin, got us great Yankees tickets!"

The key to understanding commas around that and which clauses (aka relative clauses) is to extend that thought about appositives to encompass those clauses. If the clause is essential, if it selects out of a larger group, NO commas separate it from the sentence. If the clause merely renames or adds information, put commas around it. "The shoes that I bought on Fifth Avenue cost $400. The shoes that I bought on 14th Street cost $20." The clauses "that I bought on [street name]" select certain shoes from out of a much larger, possibly Imelda Marcos-like collection of many forms of footwear. It is essential. "The room, which had been painted a terrible gray, was repainted a cheerful yellow." The which clause just gives some additional info here.

And what about those that's and which's?! The words that and which in clauses are relative pronouns.* They take the place of the noun that immediately precedes them, and they relate the clause to the rest of the sentence.

Now comes the tricky part, and the peculiarly American witchery: When the clause is essential, when it does not have commas, Americans use "that." When the clause is nonessential, and the information is renaming or additional and takes commas, Americans use "which."

The British, and those taught by the British (much of the postcolonial world English-speaking population, including some of the Caribbean and African nations, Australia, Canada, and the rest of the former Commonwealth, as well as the rest of the UK, such as Ireland and Scotland), well, those folks don't pay any attention to this distinction at all. Not at all, and blissfully, which's and that's appear, sans commas, wherever they may.

That said, I find it hard to declare which is right or wrong. The usage is contextual, depending on the surrounding culture. Perhaps the American usage is more precise; perhaps it is more confusing. But when in the USA, do as the Americans do. Early in my career, I made a nice little bit of cash Americanizing British crime novels, changes which's to that's, colour to color, and jumpers to sweaters. A little knowledge can be dangerous, yes, but enough of it can also be a cash cow. Observe the distinction!


*("That" does other things in other situations, of course.)