I’ve been a grammar geek for a long time. Diagramming sentences was something I used to do for fun in school. (No, seriously.) In particular, I can remember reveling in the longish Latinate terms I encountered in my junior-high English textbooks: terms like predicate nominative and objective complement and correlative conjunction. Especially correlative conjunction. The word correlative is just fun to say.

We use correlative conjunctions all the time. They’re pairs of words — like both . . . and and either . . . or — which connect parallel items. Prose writers find them useful when they’re building complex sentences. In my mind, they help readers navigate trickier syntactic paths, kind of like those colored symbols that mark hiking trails.

Both . . . and is probably the most common correlative conjunction, but it does come with one stipulation. It can be used only with two items — not three, not four, not more. The correct format is “both A and B.” I know, I know: this sounds obvious and intuitive. But you might be shocked at how often I encounter a statement like this one:

These trends affected both our selection of case studies, our recommendations, and our strategy.

See the problem? The basic construction here is “both A, B, and C.” And that doesn’t work, since both by its very definition refers to a pair of things. Fix the problem either by deleting both . . .

These trends affected our selection of case studies, our recommendations, and our strategy.

. . . or by reducing the number of items to two.

These trends affected both our recommendations and our strategy.

It’s that simple. And in case you were wondering, some correlative conjunctions work just fine — even elegantly — with more than two items.