A parenthetical element is information that is nonessential to the meaning of a sentence, such as an example, a clarification, or an aside. This type of sentence component may include the following types of clauses and phrases, as long as the information is nonrestrictive:
- Relative clauses, which commonly begin with which, who/whom/whose, where, or when
- Appositives, or nouns or phrases that rename preceding nouns or phrases
- Participial phrases, or verb-based phrases that describe preceding nouns
- Prepositional phrases, or preposition-based phrases that often describe preceding nouns
- Phrases beginning with such as, including, e.g., or i.e.
The supplementary information provided by a parenthetical element is typically enclosed by two commas, parentheses, or dashes, separating the nonessential material from the rest of the text. Although these punctuation marks have a common purpose, the content of their associated text and their level of emphasis of this text may vary.
Commas are frequently used as a slight interruption, as in the following examples:
- The experiment, which was performed in triplicate, yielded significant results. (relative clause)
- The experiment, the first of its kind, yielded significant results. (appositive)
- The experiment, requiring months of planning, yielded significant results. (participial phrase)
- The experiment, after months of planning, yielded significant results. (prepositional phrase)
- The experiment, including three replicates and two controls, yielded significant results. (“including” phrase)
In each of these sentences, the phrase bracketed by commas is the parenthetical element. In other words, the sentence “The experiment yielded significant results” maintains its meaning when the parenthetical element is removed.
Note that in the cases of e.g. and i.e., not only the parenthetical information but also the abbreviation itself should be enclosed by commas, as in “The main limitations of this study, i.e., the small sample size and the lack of a positive control, should be noted.”
Similar to commas, parentheses separate nonessential information from the rest of the sentence. However, this punctuation emphasizes its content slightly more than commas do, as in “The experiment (the first of its kind) yielded significant results.” Moreover, parentheses do not require a specific grammatical connection to the rest of the sentence, as demonstrated below:
- The experiment yielded significant results (p<0.05) after the inhibitor was added. (data)
- The experiment yielded significant results (see Table 1 for more information) after the inhibitor was added. (independent clause)
Note that in these examples, the parenthetical information does not fall into any of the categories of common nonrestrictive elements defined above.
Dashes are typically used to strongly emphasize a parenthetical element, even more than parentheses do, often conveying a sudden change in tone or content. For instance, an appositive is highlighted by dashes in the following sentence: “This result—an unforeseen consequence of our technique—motivated us to modify the protocol.” Although commas or parentheses would also be grammatically correct here, the dashes reinforce the unexpected nature of the finding. However, note that in scientific writing, commas and parentheses are more frequently used than dashes, which are sometimes construed as informal.
Dashes may also be used to set off a parenthetical element containing commas, as in the following example: “The main limitations of this study—including the small sample size, the lack of a positive control, and its regional focus—should be noted.”
How to choose?
The decision of whether to use a pair of commas, parentheses, or dashes is often a judgment call, dependent on the perceived importance of the information being communicated. Generally, as William Safire wittily wrote, “Put in olive-sizing terms, commas are large, parentheses giant and dashes supercolossal.” However, keep in mind that although these punctuation marks can add important clarifying information to a sentence, their overuse can be distracting to the reader, interrupting the flow of ideas. As in many areas of life, moderation is best.
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