What Are Those Lines on a Car’s Rear Windshield?

Although they don’t take up a lot of square footage, cars harbor a surprising number of minor mysteries. People have wondered about everything from what symbols on dashboards mean to the meaning behind the ‘A’ button to why grab handles are above the doors.

Another unexplained feature: Horizontal lines running across rear windshields. Why do cars need them? Do some cars not have them? And do they even have a name?

The Reason Cars Have Lines on the Rear Windshield

According to Motor Biscuit, the lines you see on a vehicle’s rear windshield are actually called secondary defrosters. The metal and resin wires placed on the glass carry a current and emit heat when the rear defroster is activated, so the windshield is quickly cleared of frost or condensation.

The secondary defrosters aren’t part of your vehicle’s primary heating and venting system, which relies on dry, warm air from vents to defog the front glass. The rear windshield doesn’t need the vents; the heating mechanisms are directly on the surface of the glass. As a result, defrosting happens much more quickly.

Why Don’t All the Windows Have Lines?

Not having to wait to defrost the rest of the car or scrape ice and snow off the side windows sounds preferable to trudging around outside in bad weather. However, secondary defrosters aren’t found on front windshields or the sides for two valid reasons: visibility and practicality.

While the lines on the rear windshield may obscure your view to some degree, most driving is forward-facing. Lines on a surface that you use for the vast majority of your time inside a vehicle might prove distracting.

It’s also a matter of design on the front windshield: Front glass is typically multi-layered to reduce glass shards in case of a collision. While putting secondary defrosters in isn’t impossible, it’s more challenging than the comparatively thinner rear glass.

Still, manufacturers have tried. The 1986 Ford Taurus had a heated front windshield, which Ford dubbed Insta-Clear. (It used the wire conduction system.) Many luxury cars, including Porsche, Jaguar, and Rolls-Royce, boast of heated front windows, which use either very fine, conductive wire or a conductive film embedded in the glass, reducing or eliminating the visibility issues with the kind of wire used for rear glass.

But because the need (or want) for heated glass is mostly relegated to colder climates, carmakers likely struggle with the expense and whether it provides any overall benefit. For the end user, there’s also the issue of what to do if the heating elements fail. In most cases, you’d have to purchase a brand-new windshield.

Heated front windshields seem to be more popular and common in Europe; in the United States, they remain largely a feature in high-end vehicles. Unless a more cost-effective solution presents itself, you’ll probably be stuck scraping ice off your car for the foreseeable future.

Read More About Cars:

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.