If you’re about to offer a piece of information that’s crucial to your sentence, offer it up without a comma and with a “that”: Please fetch me the Bible that’s on the table. Which is to say: Fetch me the Bible that is on the table rather than the Bible that’s under the couch or the Bible that’s poised picturesquely on the window seat. If you’re offering a piece of information that’s perhaps interesting amplification but might well be deleted without harm, offer it up with a comma and a “which”: Please fetch me the Bible, which is on the table. One Bible and one Bible only.
