Why is there a line on the left rearview mirror?

[AuroraDella]

This line is not merely for aesthetics or personalization; it has its own technical name—"dual-curvature rearview mirror." Its primary function is to provide the driver with a wider field of view behind the vehicle. The specific principle is relatively complex, but in simple terms, it involves the radius of curvature. More relevant information is as follows: Note 1: The section of the dotted line closer to the inner side of the vehicle body provides the driver with a normal field of view with a larger radius, while the section farther from the vehicle body has a smaller radius. This design effectively reduces blind spots in the driver's rearview mirror, significantly lowering the probability of accidents. Note 2: The presence of this line allows the driver to see vehicles behind more clearly when making U-turns or lane changes. Many modern vehicles now use special materials for this dotted line on the rearview mirror, enhancing visibility during nighttime driving. Some vehicles without proper lighting or with overly bright headlights can obscure the road, but this line on the rearview mirror improves clarity.

[EliAnn]

Last time I accompanied a friend to pick up a new car, I was also puzzled by this. The vertical line on the left rearview mirror is actually a safety feature called a dual-curvature dividing line. The outer part of the mirror beyond the line has a greater curvature, allowing you to see a wider view of the road and reducing blind spots when changing lanes. The inner part is a regular mirror, providing a more accurate distance of vehicles behind you. This design solves an old problem—previously, when changing lanes, you always worried about a car suddenly appearing from the side. Now, if you catch a glimpse of a car approaching in the outer area of the line, you’ll be alerted. In models with heated rearview mirrors, this line also hides heating wires. Got it?

[Luis]

I've been asked this question many times while repairing cars. That vertical line is not a flaw, but a blind spot monitoring zone specifically designed by the manufacturer. Simply put, the mirror is divided into two parts: the inner section near the car body provides a normal viewing distance, while the outer section uses a spherical distortion treatment to widen your field of vision by nearly 30%, making it particularly suitable for checking the diagonal rear when changing lanes on highways. The U.S. has long mandated the use of such mirrors. Remember to align the vertical line with the rear door handle when adjusting the side mirror, so both the inner and outer zones can function at their best, providing the most scientific coverage of your field of vision.

[VonColton]

When I first started driving, I also thought the mirror was cracked! Later, I learned it's actually a curvature dividing line. The principle is similar to progressive lenses in glasses—the area closer to the driver maintains normal visibility, while the outer section is curved to expand the view by an additional lane. Many car models now add an anti-glare coating to this area, which is especially effective when vehicles behind use high beams at night. One important note: avoid spraying glass cleaner directly on this section, as it can damage the special coating—just wipe it gently with a damp cloth.

[SanChloe]

This is a common question from beginners during car reviews. That white line is actually the boundary between two types of lens materials. The inner flat mirror ensures true imaging, while the outer convex mirror expands the field of view. Data shows this design reduces lane-changing accidents by 15%. The adjustment trick is to keep the rear window occupying 1/4 of the inner mirror surface, with the outer part just showing the wheel position of the adjacent lane. Some car owners replace the original mirrors with ordinary glass during modifications, which actually increases blind spots and creates hazards—it's really unnecessary to tamper with this safety feature.