Better sentence-writing in 30 minutes a day - Dianna Campbell 1995

The difference between which and that
Using commas in embedded sentences
Punctuating sentence combinations

When a sentence contains an embedded clause beginning with which, it usually calls for two commas because which is normally used with extra information that is not needed to make the subject clear and specific.

When an embedded clause begins with that, the information in the clause is usually needed to understand the subject, so no commas are used. Study these examples:

(a) Neverland, which is often mistakenly called “Never-Never Land,” is the place where the children in Peter Pan met mermaids, pirates, and Indians.

(b) The lines that connect points of equal barometric pressure on a map are isobars.