Answer: When is it appropriate to use "which" vs. "that"?

[katiefoolery.livejournal.com]

t_verano poses an interesting question: When is it appropriate to use "which" vs. "that"? What is the appropriate comma use in each situation?

Is there, in fact, a distinct rule that applies to this situation? Or does it depend entirely on mood, which can change from one moment to the next?

( Let's find out, with examples from Discworld... )