I first encountered this term just a few years ago, in Frank Kermode's chapter on Hamlet in Shakespeare's Language--but when I did I realized immediately that it was a defining feature not only of most of the works I wrote my dissertation on, but also of my own writing. Literally "one from two," hendiadys refers to a pair of words linked by "and" that expresses a single meaning neither word alone conveys.* Some of these we use every day, unthinkingly, to the point that they're clichés: "house and home," "law and order," "doom and gloom." Ideally, though, they're less common, more evocative, and even surprising pairs.
I'd like to claim that I use hendiadys consciously (and of course evocatively!), but I notice it most when when I'm drafting--when I let myself just write, just in order to get words on the page and without turning on the editorial function. When I do that, I write in pairs unthinkingly and almost compulsively.
Here are two sentences I just added to the essay I'm working on:
In this passage, [Thing A] is characterized by its fragmentation and imperfection. Unlike [Thing B], which appears functional and whole while being divorced from any authentic source of motion or meaning, [Thing A]'s brokenness is a sign of its legitimacy.Fragmentation and imperfection; functional and whole; motion or meaning--and actually there's another, ghost pair, "legitimacy and truth," which I rejected a split second before typing it.
So what the hell is that all about? Now, it may simply be that I believe in the buddy system when it comes to word usage (why settle for just one when two are available?), but since I don't tend to verbal excess or floweriness in other ways, I think my use of hendiadys is at least partly a process of refinement: of figuring out whatever it is that I really mean and how to say it.
In the above example, for instance, I'm still not sure whether what I'm talking about is motion or meaning. . . or some third and entirely different idea; that pair would definitely go in a revision. Similarly, I pre-rejected "legitimacy and truth" because, although the two terms don't express the same idea, they're too close in sense and not really interesting as a unit anyway. When I generate that kind of pair, it's clearly just a placeholder: two words that are different approximations of what I think I'm trying to say. On the other hand, pairings like "fragmentation and imperfection" or "functional and whole" do useful work, conveying ideas that no single term can express.
So partly it's that I don't like to commit--I'd rather generate possibilities than foreclose them--but it's also about what sounds right in my head: pairs and trios of words and terms and clauses just make rhythmic sense to me in ways I can't explain.**
*Technically, hendiadys has a more narrow meaning, referring to pairs in which one noun is clearly subordinate to the other and could be converted into an adjective modifying it--but this broader usage is approved by at least some of my reference books, so I'm going with it.
**Before writing this post I was unaware that there is also such a thing as hendiatris, which involves three words that express a single meaning. But you'd better believe I employ that, too.