When I was in grade school, English class became the Language Arts. None of us really knew why, but many of our parents weren’t happy: just get on with teaching them the basics, they said, so they will know how to read and write. The change, though, may have harbored a defensible reason, and one that can help us write better.
To study a language means more than mastering its basics of grammar. It also means studying logic and rhetoric, which in less grandiose terms we can call critical thinking and style. Grammar, critical thinking, and style (which together are traditionally called the trivium) can never really be separated; we can isolate them and study each extensively on its own, but when we sit down to practice these three arts of language, we have to balance and orchestrate them in every sentence we write.
One place where the art of logic is demonstrated is in what grammar calls apposition. If I tell you, for example, that my friend Tom is a lawyer, the substance of what I want to communicate, because it is the logical subject of the clause, is that my friend is a lawyer; the proper noun Tom then immediately follows friend in order to identify the particular friend I’m talking about, on the assumption (sometimes precarious, I’ll admit) that I have more than one friend. The term apposition, derived from Latin, means exactly that: put next to.
The critical thinking is seen here in the way in which I attach Tom to friend in the sentence. My purpose is to define the friend I’m referring to, so I place the name Tom immediately after friend, with no commas intervening. By thus cementing the two words together, I restrict you, my reader, from thinking in this context about any other friend I might have than Tom. This is called, appropriately enough, restrictive apposition.
But what happens if the scene changes and I tell you, Chicago, a major midwestern city, sits on Lake Michigan. Now my purpose can’t be to define Chicago, but rather simply to describe it more fully; to do this, I isolate the amplifying phrase, a major midwestern city, with commas. And because there is no need here to restrict the reader from thinking about which Chicago I’m referring to (for in the context there is only one), this construction is called nonrestrictive apposition.
Restrictive apposition, then, defines a term and never uses commas, and nonrestrictive apposition describes a term and always uses commas. This terminology might appear arcane, but because it involves making clear distinctions, which is the work of logic, apposition is concerned with one of the important requirements of well-written English.
It was the British author C.S. Lewis (there it is again: C.S. Lewis is in restrictive apposition to British author because there are certainly many British authors) who said that grammar teaches us how to talk and logic teaches us how to talk sense. These two arts of language, together with style, work in complement whenever we write and speak with force and clarity.