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Here is an example sentence:

At the start of the day you may (if you like) make yourself a slice of toast.

Are the brackets appropriate to separate the clause "if you like" from the main sentence? Is there a fundamental difference between the following alternatives?

At the start of the day you may, if you like, make yourself a slice of toast. (commas)
At the start of the day you may — if you like — make yourself a slice of toast. (em-dashes)

Are there rules which state when each should be used?

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3 Answers 3

up vote 2 down vote accepted

Commas are used to set off parenthetical elements if a slight pause is intended. They help set off the parenthetical element without causing a break in narrative. Em dashes or parentheses are used if a longer pause is intended.

Phrases or clauses in parentheses seem less closely related to the sentence than those set off by commas. Em dashes usually set off amplifying statements or asides.

At the start of the day you may, if you like, make yourself a slice of toast.

At the start of the day you may make yourself a slice of toast (only if you haven't eaten anything the previous evening of course).

At the start of the day you may--now don't go spreading any butter on it--make yourself a slice of toast.

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Perfect, thanks! –  Andy F Feb 11 '11 at 9:45

I think of there being a useful distinction to be made among three roles of parenthetical statements: amplification, clarification and interjection. Although there are definitely no hard-and-fast rules on this, I tend to think they line up as follows:

  • amplification: commas or parentheses
  • clarification: commas
  • interjection: em dashes

I would argue that the commas work best in the original example, because the intent is to clarify the optional nature of the toast. You don't have to have toast, you may have toast, but only if you want toast. The commas signal that this parenthetical thought is still closely related to the rest of the sentence. At the other extreme would be a true interjection, much less connected to the main thought:

At the start of the day you may -- perhaps if you are in the mood to venerate the great radio comedians Bob and Ray and their "House of Toast" skit -- make yourself a slice of toast.

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I think that visual effect comes into play, as well. Dashes seem so jarring to me. Of course, there's a bit of a chicken-or-the-egg aspect to that: perhaps the dashes seem jarring because they typically denote a more significant jump from the idea of the main sentence? In any case, I notice that I tend to use only commas or dashes when writing marketing brochures. Somehow parenthesis look less professional to me. I certainly haven't helped as much as previous responders who provided cut-and-dry classifications, but I do think the visual element is a factor worth considering.

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