punctuation - Coordinating conjunction immediately followed by parenthetical — Is a comma needed or not?

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?

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.

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  Cleopatra appeared poised and composed. But on the inside, she was seething with rage. — Simple adverbial phrase at the beginning of a sentence.

  
  


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  Her only option was to support Mark Anthony once again. But, if he should perish, it would be the end of her. — Not a simple adverbial phrase but a clause.

  
  


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  She knew her cause was lost and, in a fit of final despair, had a serpent brought in to extinguish her life with its venom. — Not at the beginning of a sentence.

  
  

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.

Laxity once introduced, however, has to be carefully kept within bounds. It may be first laid down absolutely that when an adverbial clause is to be stopped, but incompletely stopped, the omitted stop must always be the one at the beginning, and never the one at the end. Transgression of this is quite intolerable; we shall give several instances at the end of the section to impress the fact. But it is also true that even the omission of the beginning comma looks more and more slovenly the further we get from the type of our above cited sentence. The quotations immediately following are arranged from the less to the more slovenly.

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  His health gave way, and at the age of fifty-six, he died prematurely in harness at Quetta.—Times.

  
  


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  If mankind was in the condition of believing nothing, and without a bias in any particular direction, was merely on the look-out for some legitimate creed, it would not, I conceive, be possible...—Balfour.

  
  


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  The party then, consisted of a man and his wife, of his mother-in-law and his sister.—F. M. Crawford.

  
  


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  These men in their honorary capacity, already have sufficient work to perform.—Guernsey Evening Press.

  
  

It will be observed that in the sentence from Mr. Balfour the chief objection to omitting the comma between and and without is that we are taken off on a false scent, it being natural at first to suppose that we are to supply was again; this can only happen when we are in the middle of a sentence, and not at the beginning as in the pattern Cranmer sentence.