Proper Punctuation for Appositive Phrases
When two terms that mean the same thing are introduced in succession in a sentence, careless writers, by omitting a crucial comma, often imply that the definition of the terms is in fact distinct. Here are some examples of misleading statements, plus discussions and revisions.
1. “Sunlight is a mixture of different colors or wavelengths, which combine to form white light.”
Colors and wavelengths are, for practical purposes, the same thing, but “colors or wavelengths” implies otherwise. To signal that wavelengths is an alternative term for colors, it should be set off in an appositive phrase: “Sunlight is a mixture of colors, or wavelengths, that combine to form white light.” (Notice, too, the replacement of which with that and the deletion of the comma that followed it — not all colors, or wavelengths, combine to form white light; only these do. I also deleted the usually superfluous adjective different.)
2. “Its odd properties are essential for the evolution and survival of life on Earth, particularly given its ability to form a weak connection called a hydrogen or H-bond.”
The weak connection is not called a hydrogen or H-bond, and it does not have the alternative names hydrogen and H-bond, both of which possibilities are suggested by this statement; the choices are “hydrogen bond” and H-bond.
To indicate that H-bond is a distinct term, that it is not an alternative to hydrogen alone, and that it is an abbreviation of “hydrogen bond” requires a minimal pair of corrections: the use of the full phrase “hydrogen bond” and the insertion of a comma after that phrase. The revision is “Its odd properties are essential for the evolution and survival of life on Earth, particularly given its ability to form a weak connection called a hydrogen bond, or H-bond.”
3. “Then, as that part of Earth passes out of the gravitational bulge, the tide goes out or ebbs.”
Again, the lack of a comma between two terms separated by or implies distinct meanings (suggesting that going out and ebbing are different actions), but this sentence, in describing a scientific phenomenon, explains a process and then supplies a perhaps unfamiliar synonym, so a comma should divide them.
But there’s another problem — one that I haven’t seen discussed in writing guides or grammar handbooks but that has always bothered me: Why, when introducing a new term, supply the better-known synonym or a definition first — what’s the use of including the new term if it’s not presented before the aid to comprehension?
It seems more logical to provide the new term first, then provide context: “Then, as that part of Earth passes out of the gravitational bulge, the tide ebbs, or goes out.” (The second example in this post at least positions the appositive terms sensibly, and the first example does not apply, as one term does not define the other, as here, or clarify it, as in the second example.)
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