Separate parenthetical expressions with commas.
Definition of Parenthetical Expression. These are expressions that do not add essential content for understanding the sentence, such as an off-topic comment or a phrase inserted in a sentence that breaks the flow of the idea. These expressions may be placed in parentheses; hence the name.
Rule AA is something of a catch-all, a grammatical version of “other duties as assigned.” Many phrases and clauses are considered parenthetical expressions, including appositives, direct addresses, interpolated asides, and interjections.
Basically, any expression, description, comment, etc. that interrupts the flow of ideas, that can be moved around in the sentence, and that can be placed in parentheses without confusing the reader needs to be separated from the rest of the sentence with commas.
Sample 12.1. The new mall, I have heard, will be huge.
In sample 12.1, the parenthetical expression is I have heard. This is not part of the idea being expressed in the sentence. It can be moved to the front or end of the sentence. And it could be placed in parentheses. As such, it is separated from the rest of the sentence with commas, one before and one after. Also, if I had written it at the end of the sentence, I would still need to separate it from the rest of the sentence.
Sample 12.2. This economic forecast model, compared to other models, shows flat growth.
In sample 12.2, the parenthetical expression is compared to other models. Wherever I put it in the sentence, it will need to be separated by commas.
Sample 12.4. Compared to other models, this economic forecast model shows flat growth.
Sample 12.5. This economic forecast model shows flat growth, compared to other models.
Sample 12.4 uses the parenthetical expression as an introductory adverbial phrase (Rule G), and sample 12.5 uses it as a non-grammatical final description (Rule X). Because it is a parenthetical expression, no matter where it is in the sentence, it needs to be separated from the rest of the sentence with commas.
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