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commas with parenthetic expressions

Parenthetic expressions are non-restrictive and therefore require commas:

  • We could see that the plan, if not actually rejected out of hand, was far from popular with senior management.

A common error occurs with parenthetic phrases following the conjunction that. The comma that belongs after the conjunction is often placed before it instead:

  • Incorrect: The odd thing was, that no matter how he tried, he couldn’t remember where he had left the document.
  • Correct: The odd thing was that, no matter how he tried, he couldn’t remember where he had left the document.

Ensuring correct sentence structure with parenthetic expressions

If a parenthetic expression is removed from the sentence, the remainder of the sentence should read as a coherent, grammatically correct whole.

For example, the following sentence is incorrect:

  • The task force wanted to show that it was as good, if not better, than its predecessors.

The expression “as good” needs to be followed by the conjunction “as,” not “than." Therefore, if we remove the parenthetic expression “if not better,” we will not have a grammatical sentence:

  • Incorrect: The task force wanted to show that it was as good than its predecessors.

The original sentence should be recast as follows:

  • Correct: … it was as good as, if not better than, its predecessors.

Omitting one or both commas

Occasionally it may be expedient to omit the first of the pair of commas around a parenthetic expression:

  • But without realizing it, he had sparked a whole new controversy.

The parenthetic phrase here is "without realizing it."

Both commas can sometimes be safely omitted; under no circumstances, however, should the second comma be omitted while the first is retained:

But without realizing it he had sparked a whole new controversy.

not

  • But, without realizing it he had sparked a whole new controversy.

Using parentheses or dashes around parenthetic expressions

Parenthetic expressions may be set off by parentheses or dashes instead of commas, depending on the degree of emphasis or pause desired, or the length of the expression. Compare:

  • Jane (evidently) had no stake in seeing the dispute continue.
  • Jane evidently had no stake in seeing the dispute continue.
  • Jane, evidently, had no stake in seeing the dispute continue.
  • Jane—evidently—had no stake in seeing the dispute continue.