> As I sit here, looking aimlessly into space, I think to myself... why do mirrors only reflect in the horizontal plane? Why not the vertical plane as well?
>
> Hmmm...
>
> Chr"No doubt the answer is obvious, but ah well..."ico

Essentially it's because when we turn to look behind us, we're rotating our perspective about the vertical axis. Rotating half-way about the vertical axis flips the horizontal orientation of two objects but preserves the vertical orientation.

Looking into a mirror is seeing this rotated perspective without actually rotating our heads.

As a side note, note that beyond being a parallel line to the mirror plane, the preferred axis here is dependent upon the observer, not the mirror itself.

--gremlinn