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.

Source: http://www.npr.org/sections/ed/2014/08/22/341898975/a-picture-of-language-the-fading-art-of-diagramming-sentences