William Strunk's Rules of Usage states:
If a dependent clause, or an introductory phrase requiring to be set off by a comma, precedes the second independent clause, no comma is needed after the conjunction.
Strunk's example illustrates the point:
The situation is perilous, but if we are prepared to act promptly, there is still one chance of escape.
However, I would be tempted to put a comma after the "but," like this:
The situation is perilous, but, if we are prepared to act promptly, there is still one chance of escape.
So, I know you're thinking, "Well, Strunk's Rules of Usage says that you're wrong. No comma."
But is Rules of Usage really the best source? I'd like to see this rule in a more contemporary and authoritative source, like the Chicago Manual of Style.
There are many instances — such as in The New York Times — where the comma is included before a parenthetical comment that comes after a coordinating conjunction.
Any authoritative source on the issue?
Best Answer
Ordinarily parenthesis should be marked with commas on both sides. But Fowler recommends that the first comma should be left out after a coordinating conjunction if what follows is a simple adverbial phrase—if I interpret him correctly. (Coordinating conjunctions are and, or, but, and possibly some semi-conjunctive adverbs like so, therefore, however, etc.) The comma should be retained before a subordinate clause or when its omission would cause ambiguity. This probably includes conjunctions that are not at the beginning of a sentence. Fowler's advice seems balanced and practical.
The relevant text from Fowler's The King's English follows. He is arguing for general laxity with adverbial phrases, and would generally allow both commas and no commas around them. He goes on to say that, if commas are used, one must never omit one but write the other; however, he gives the above rule as an exception, in order to avoid an abundance of commas, logical though they might be.