A simple lesson about the strength of the mighty little comma can be learned by noting the difference in meaning that results depending on its presence or absence at the end of a parenthetical phrase.
More specifically, in the types of sentences shown below, when an attributive phrase—one that identifies the source of the information provided in the framing sentence—is inserted in the midst of the sentence and a comma precedes the interjection but none follows it, what is said is often not equivalent to what is meant:
1. “Every nine years, it was decreed that the fragment must be conveyed to another place of sanctuary.”
The point of this sentence is that an action is described as having occurred every nine years, and that this action was decreed. That latter detail is the content of the attributive phrase. But without a comma closing the interjection, the implication is that the decree was issued every nine years.
However, what the sentence means is that a decree was issued requiring the action to occur every nine years—that’s a much different idea, and this slightly revised sentence correctly expresses it: “Every nine years, it was decreed, the fragment must be conveyed to another place of sanctuary.”