Em dashes are handy little items for setting phrases apart for special attention, but be cautious when employing them, because when misused, they can obscure rather than assist in comprehension.
1. "For the most part, this water comes from aquifers—that's groundwater—or from surface waters—that is, rivers and lakes."
When em dashes come in pairs, what lies between is a parenthetical digression that merits a more dramatic break than that indicated by a brace of commas or two parentheses. If the parenthetical phrase ends the sentence, however, only a single em dash is needed.
Three or more em dashes in one sentence creates an ambivalence in the sentence structure. In this case, it's better to use parentheses—and to avoid mixing em dashes and parentheses for digressions of equal or parallel impact, use them for the second digression as well: "For the most part, this water comes from aquifers (that's groundwater) or from surface waters (that is, rivers and lakes)."
2. "Her recent roles have shown her interest—and her ability—to go beyond the usual popular movie."
Be careful that when a phrase is parenthesized, what precedes and follows it is grammatically sound: "Her recent roles have shown her interest in going—and her ability to go—beyond the usual popular movie."
3. "The collapses could play out in the seven states that rely on the Colorado River and its tributaries—Arizona, California, Colorado, Nevada, New Mexico, Utah, and Wyoming—as ever-increasing water use, ever-growing population and a changing climate shrink the flow."
If the parenthetical delineates a list or the parts of a whole, as here, the opening em dash should immediately follow the whole: "The collapses could play out in the seven states—Arizona, California, Colorado, Nevada, New Mexico, Utah, and Wyoming—that rely on the Colorado River and its tributaries as ever-increasing water use, ever-growing population and a changing climate shrink the flow." (Otherwise, the sentence identifies the states as tributaries.)
4. "There may be a decrease in prices—but incomes are rising—so that outcome may not happen."
When you use an em dash, you should know what you're getting yourself into. In this sentence, the writer meant to set off the entire second clause, not just the parenthetical, which is bereft without the phrase following the second em dash: "There may be a decrease in prices—but incomes are rising, so that outcome may not happen."
5. "Maybe it's just because no matter how many people have been through here—the space remains the same, seemingly untouched by human hands."
By the same token, many sentences simply don't merit even a single em dash—there's nothing to mark off for emphasis. Perhaps the writer meant to place the em dash in lieu of the comma after same, rather than the one following here, but commas suffice in both positions: "Maybe it's just because no matter how many people have been through here, the space remains the same, seemingly untouched by human hands."
This article originally ran on DailyWritingTips.com.