Final answer:
The textbook suggests using em dashes to emphasize or set off phrases in a sentence. Dashes can emphasize important appositive phrases or ideas, indicate a pause or shift, and they create a stronger break than commas.
Step-by-step explanation:
The textbook suggests using em dashes to set off or emphasize phrases within a sentence. Em dashes draw more attention to the enclosed content than commas or parentheses, adding emphasis to the appositive phrase or an important idea.
When an appositive phrase contains commas, an em dash can be useful for clarity. For example, 'The U.S.S. Constitution became known as "Old Ironsides" during the War of 1812—during which the cannonballs fired from the British H.M.S.
Guerriere merely bounced off the sides of the Constitution.' In this sentence, the em dash sets off the phrase that follows, which provides significant additional information.
Furthermore, em dashes may be used to indicate a pause, shift in thought, or contrast within a sentence, providing a stronger break than a comma would.
For instance, in the sentence 'To some of you, my proposals may seem radical—even revolutionary,' the dash adds dramatic emphasis to the adjective 'revolutionary.'
Em dashes are versatile; they can also replace other punctuation like semicolons if there's a need to make a strong connection between two clauses, as shown in the example of the Astroturf-covered car: 'The roof of that car was covered in Astroturf—a strange sight!'
Moreover, parentheses serve a subtler role, enclosing material that is supplementary or clarifying but not essential to the main message, such as dates, extra information, or sources, thereby de-emphasizing the content compared to em dashes.
An important distinction to make is that while both dashes and parentheses can enclose nonessential information, dashes amplify importance while parentheses minimize it.
In summary, it is essential to choose between em dashes and parentheses depending on whether you intend to draw the reader's attention to or away from the enclosed material.