If your car has a traditional manual rearview mirror, you may have noticed a small tab or lever sticking out from the bottom. Many drivers see it every day without ever using it, while others may have moved it accidentally and wondered why the reflection suddenly changed.
That little piece isn’t decorative. On many manual rearview mirrors, it operates a simple day/night function designed to reduce the glare you see from headlights behind you while driving after dark.
It’s an old, remarkably simple piece of automotive engineering—and understanding how it works can make nighttime driving more comfortable.
What Is the Tab Under the Rearview Mirror?
The small lever is commonly found on manual day/night rearview mirrors. Moving it changes the angle of the mirror assembly so that the driver sees a much dimmer reflection instead of the normal bright one.
During the day, you generally want the mirror in its normal position because it provides a clear view through the rear window.
At night, however, bright headlights from a vehicle behind you can create uncomfortable glare in the mirror. That’s when the second position becomes useful.
What Happens When You Move the Tab?
Try it while the vehicle is safely parked and you’ll notice something interesting.
Moving the tab doesn’t simply cover the mirror with darker material. Instead, the mirror changes position slightly and presents a dimmer secondary reflection to your eyes.
The headlights behind you become much less intense, but you can still see enough of the scene behind the vehicle to use the mirror.
Move the lever back and the normal brighter reflection returns.
How Can a Simple Lever Make the Reflection Darker?
A traditional manual day/night mirror uses clever optical design rather than electronics.
The mirror assembly can provide two different reflections. The strong reflection is the one you normally use during daylight. A much weaker reflection can come from the front surface of the mirror glass.
When the lever changes the mirror’s angle, the strong reflection is directed away from your eyes while the weaker reflection becomes the one you see.
That’s why headlights suddenly appear much dimmer even though the glass itself hasn’t magically become tinted.
And there is another detail many drivers don’t realize: not every modern car uses this little lever anymore.
