Certain prose rules are essentially inarguable—that a sentence’s subject and its verb should agree in number, for instance. Or that in a “not only x but y” construction, the x and the y must be parallel elements. (More on this in Chapter 6: A Little Grammar Is a Dangerous Thing.) Why? I suppose because they’re firmly entrenched, because no one cares to argue with them, and because they aid us in using our words to their preeminent purpose: to communicate clearly with our readers. Let’s call these reasons the Four C’s, shall we? Convention. Consensus. Clarity. Comprehension.
I can't wrap my head around this one, so I'll follow CMOS and MW.
