Consider this sentence:
You may worry about the Fed raising interest rates, or a market meltdown, but these risks should not change your investment plans.
Could the comma before "or" be omitted? I could argue no, because then the sentence would imply the reader may worry about the Fed "raising a market meltdown," which is clearly not the intent (A market meltdown is a separate risk). On the other hand, the phrase "raising a market meltdown" is clearly nonsensical, hence a comma may not be needed to clarify the writer's intent (the Fed could "cause" a market meltdown, but not "raise" one).
In other words, is it acceptable to omit a comma when its absence creates a syntactical ambiguity, but not a semantic ambiguity?
Rephrasing is usually the best answer to these kinds of cases. It's a made-up example. Assume the sentence as phrased is my best option, and my only decision is comma or no comma.
My understanding is this:
- If a comma is optional, take it out.
- However, if there is any possibility of ambiguity (without the comma), leave it in.
Ideally, I'm looking for an answer that cites an authoritative source (e.g. Chicago Manual of Style).