Here is one of the questions asked about the intro prepositional phrase and the comma, and here is my answer. We were talking about a short prepositional phrase at the beginning of the sentence that is a simple modifier. That prepositional phrase does not need a comma.
What about those instances where a short prepositional [phrase] at the beginning of a sentence, without a following comma, has the effect of creating needless ambiguity?
Q. When did this happen, and who was in charge?
A. At the time, Bob was in charge. (Meaning that Bob was in charge at that time.)
A. At the time Bob was in charge. (Meaning that the event happened at the time that Bob was in charge as opposed to the time that someone besides Bob was in charge.
First, I am not sure that we can always determine this difference. We would have to have more context to nail this down. If we cannot distinguish the difference from the context, then I am for following the rule and leaving it without the comma
The rule that helps the situation is the one that says that we can insert a comma for “clarity” anywhere that it helps with the clarity of the meaning, that is, where it improves readability.
…In general, meetings were held on Fridays.
…In 2012, taxes were deferred.
So if the comma helps discern the difference in meaning here, then I am all for it. And it follows the “clarity” rule. I am always happy when we can follow rules. 🙂
Thanks for the question, Jim.