What are the lines on the rear window for?

One cold January morning, I stood in the driveway with a plastic ice scraper, my breath fogged and my fingers numb, working on the windshield as if it owed me money. When I looked back, the rear window was quietly clearing itself. The frost was melting in even, small strips, from top to bottom, leaving the windshield stubbornly frosted. I stood there for a moment, ice scraper in hand, feeling a little cheated.

That’s when I really noticed those lines on the rear window. You’ve seen them a thousand times in the mirror. Thin, evenly spaced, running from side to side. Turns out, they’re not just decoration. They’re real work.

What exactly are those lines on a car’s rear window?