A number of months ago, in a post called Apposition, I took up the question whether commas would be necessary in a sentence like this: My brother, George, is tall. Recently, I was talking with someone who tutors middle school students, and the materials he is compelled to use on the same topic were downright wrong. Instead of explaining the difference between using and not using commas, they simply instructed students always to place such amplifying words in commas. The problem, however, is more subtle than that. Understanding it will take us uphill for a time, but not up a sheer precipice.
First, it is important to remember that questions of punctuation can be matters of style or matters of logic. If you think, for example, that you would not have put a comma after the word first in the previous sentence here, that would be a stylistic matter; what is being said would not materially change, with or without that comma. If, however, you do or do not place commas in the sentence My brother, George, is tall, you change the implication of what is being said. Implication, or suggestion, is decidedly within the bailiwick of logic, and it can be difficult to argue with a bailiff.
Commas separate; that is their function, job, and duty. We separate what doesn’t belong together, or what might unduly influence something else by too close an association. In English grammar, a word or phrase or clause standing right next to another implies some connection between the two; the principle is called proximity, and that’s how sentences and paragraphs are constructed into comprehensible statements: ideally, we strive in prose to compose lines of departure, one element unfolding from the previous across a sentence until we have written out a path to an idea. There can be all kinds of twists and turns, of course, but there must be a discernible through line that makes the statement intelligible.
So if I placed commas around the name George in the sentence at hand, I would be separating that name from the noun brother immediately next to it: I would then have dissociated it, taken away its strength to define the word it stands next to, and would have consequently so weakened it in pulling it away from its brother that all it could do now would be to describe that person, not distinguish him from a number of others. Words that stand next to each other define each other; words that are isolated by commas merely describe. The former is called restrictive apposition; the latter is nonrestrictive apposition.
We might choose to describe something merely as a courtesy to our readers, or to set things up for an idea yet to appear in the paragraph. But when we define, we feel under some obligation to keep ideas clear, to preclude a misunderstanding. So if the sentence were written without commas, My brother George is tall, I am intending to define, not describe by naming, the particular brother of mine who is tall. And that, of course, would be to imply that I have more than one brother; why otherwise would I take the trouble to be so clear when no confusion could reasonably arise?
Whether or not we use commas in a sentence like this (and in countless similar sentences) is determined by the truth of the circumstances we are writing about. To explicate all those standing circumstances at every turn would overrun our readers with secondary and tertiary detail trailing off into the irrelevant, but we can imply the relevant background with what resources the grammar of English allows us. Correctly placing commas is one such resource, and in this way we can take readers through the landscape of our thoughts, not stopping to point out every rock formation, but framing their view sufficiently so that they can clearly put things into perspective.
Intelligibility is a matter of logic; things have to make a certain sense or we’re merely reading through the mistiness of someone’s inchoate thoughts. And language, thankfully, does not require that we possess as readers any special skill of divination or mind reading. Knowing the craft of writing, both its stable rules and changing conventions, will answer our rightful questions as we draft and revise and read.