Why do mirrors flip left and right but not up and down?

Mirrors don’t actually “flip left and right.” They flip front and back.

What a mirror really does

A flat mirror reverses the direction perpendicular to its surface. If the mirror is on the wall, that’s the front–back axis:

• Your left stays on the left side of the room.
• Your right stays on the right side of the room.
• Your up stays up.
• Your down stays down.
• But front becomes back (what’s closer to the mirror in real life appears “behind” the mirror in the reflection).

So why does it look like left/right got swapped?

The “left-right flip” is caused by us, not the mirror

When you compare yourself to your reflection, you usually imagine the reflection as a person facing you. The easiest way to turn yourself to face someone is to rotate around the vertical axis (like turning left/right). That rotation swaps your left and right relative to the other person.

Example:

• Raise your right hand. In the mirror, the “person” raises the hand that’s on your left side of the image.
• But that’s because you’re treating the image like another person who turned to face you—a 180° turn swaps left/right, not up/down.

If instead you rotated around a horizontal axis (a forward somersault), then up/down would appear swapped. The mirror didn’t choose left/right; your imagined rotation did.

Quick way to feel it: write on paper

Hold a sheet with text facing the mirror. The mirror makes it look backward because the page’s front/back is reversed. If you physically flip the paper to face you (turn it around), you introduce a left/right swap and now it matches what you see.

Bottom line: Mirrors reverse depth, and “left-right reversal” is a side effect of how we interpret the reflection—as if we (or the reflected person) turned around to face us.