When you think about a sentence, you usually think about words — not lines. But sentence diagramming brings geometry into grammar. If you weren't taught to diagram a sentence, this might sound a little zany. But the practice has a long — and controversial — history in U.S. schools. And while it was once commonplace, many people today don't even know what it is. So let's start with the basics. "It's a fairly simple idea," says Kitty Burns Florey, the author of Sister Bernadette's Barking Dog: The Quirky History and Lost Art of Diagramming Sentences. "I like to call it a picture of language. It really does draw a picture of what language looks like." I asked her to show me, and for an example she used the first sentence she recalls diagramming: "The dog barked."