— , ( )
Non-essential (explanatory) information
The contents of the egg shell are what matter, not the color of the outside. The shell may be commas, long dashes, or parentheses, but the explanation is inside them.
“Non-essential” refers to the sentence, not to the meaning in the story. Think “explanatory.”
Bob, who built the house, knew all its secrets.
- “who built the house” is important information for the reader, explaining why he knew all the secrets of the house.
- However, the main sentence is fine without it: Bob knew all its secrets.
Any of the three (em dash, commas, and parentheses) may be used to add explanatory information, but use them wisely and consistently.
The em dash, when adding descriptive information, implies more emphasis, perhaps because it is the most obvious. Em dashes may be used inside parentheses instead of brackets when each breaks the sentence continuity.
Commas for explanatory information will serve their purpose without interrupting the flow for the reader. Commas are better if what is between them has a close relationship to the rest of the sentence.
While these punctuations may seem to be interchangeable, care should be given in their use. Do not overuse. Consistency is important.
Comparisons of commas, long dashes, and parentheses:
- Tom, who committed the crime, confessed to the police.
- Tom—who committed the crime—confessed to the police.
- Tom (who committed the crime) confessed to the police.
- (“who committed the crime” simply dis explanatory; the phrase does not define him.)
- (Important information is added for the reader, but the main sentence does not require it.)
- The butcher, one of the three men in the tub, went out to sea.
- The butcher—one of the three men in the tub—went out to sea.
- The butcher (one of the three men in the tub) went out to sea.
- (“one of the three men in the tub” is explanatory; the phrase does not define him.)
- (Important information for the reader, identifying him, but the main sentence is fine without it.)
- Angus cows, all of which are black, are hard to tell apart.
- Angus cows—all of which are black—are hard to tell apart.
- Angus cows (all of which are black) are hard to tell apart.
- (“all of which are black” describes Angus cows without defining them.)
- (Good information for people who do not know the difference between a Hereford cow and an Angus cow, but the main sentence is complete without the explanatory phrase.)
- The houses on the island were completely destroyed by the storm, which dumped seven inches of rain in two hours.
- The houses on the island were completely destroyed by the storm (which dumped seven inches of rain in two hours).
- The houses on the island were completely destroyed by the storm—which dumped seven inches of rain in two hours.
- (“which dumped seven inches of rain in two hours” does not tell us which storm, simply what the storm did.)
- (This phrase explains how the houses were destroyed, but it does not matter to the main sentence.)
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