# Explanation of the epipolar constraint in stereo imaging

I am watching this video, in which the epipolar constraint is defined as

$$x_l \cdot (x_l \times t)=0$$

It means that the vector $$x_l$$ that passes through the observed point and the left camera origin is perpendicular to the normal of the epipolar plane, defined by that same vector, and the epipolar line.

What I fail to understand is, why is this a constraint at all?

Doesn't it hold for ANY two vectors $$v$$ and $$w$$ that $$v \cdot (v \times w) = 0$$?