What is the difference between a comma and a parenthesis? Or better, when do we set off information with a pair of commas and when do we use parentheses instead? The answer will show us why we speak more often of the art, and not the science, of writing.
The comma, of course, is one of the three chief marks of punctuation. It stands weakest in strength below the other two, the semicolon and the period, and its chief responsibility is to cut apart, or separate, elements in a sentence that need to be distinguished from one another for some reason. That could include something no more complicated than listing a series of items (I flew to Nevada, Texas, and New Mexico last year), or it could mean separating a merely descriptive addition from its noun (Airports, large or small, can be confusing places). We are often told to put a comma where we breathe, but that, I think, is much too subtle a direction to be of practical help. In the sentence I just wrote, for example, I would not have taken a breath after the word that, nor after the word think, but commas are indisputably necessary in both places.
Parentheses, likewise, mark a break, but an interruption that is more substantial and sustained in importance. It would be out of the question, for instance, to consider replacing the commas with parentheses in our earlier example: Airports (large or small) can be confusing places. The sentence is making one simple assertion, that airports can be confusing places, and the phrase large or small is meant merely to demark the subject more clearly in a practical way. The claim applies to both large airports and small ones, but instead of clumsily overwriting Large airports and small airports can be confusing places, the writer has clipped away that too precise formulation and laid down next to the noun two adjectives meant to emphasize the fact that the plural subject airports refers virtually to all airports. The two adjectives, set off correctly here in commas, highlight the meaning of the subject airports, but do not say anything additional about it.
And that’s where the difference often lies between a pair of commas and parentheses. Parentheses are felt to be more substantial than commas in separating out additional information because they imply a further assertion; they mean to say that something else entirely is being added to the clause they are interrupting. In the sentence My brother and sister, two very different people, have decided to take a vacation together this fall, the two commas that isolate the phrase two very different people, are unimpeachably correct. But here, unlike our earlier example about airports, one could replace those two commas with parentheses to wink subtly at the reader: My brother and sister (two very different people) have decided to take a vacation together this fall. The phrase two very different people abbreviates the predication they are two very different people, and that assertion is saying something completely new about the subject my brother and sister, something with an entire world of unexpected, perhaps humorous, or even zany possibilities. A new world of ideas has appeared in the midst of what the writer principally wants to say with the sentence, and the stronger parentheses bracket this world within a world in a way a pair of common commas could not.
Deciding between commas and parentheses often involves the same delicate perception we see when a cook adds a pinch of salt to the pot of soup on the stove. If I ask how much is a pinch, I betray that I believe all experience can be quantified. That, however, is not true, and so we are left in the end with just eating and reading, tasting and understanding—with experience and rules, art and science—to guide us in the right choice at the right time. Things could be worse.