projection plane

Definition 2: The vertical foot Q obtained by drawing a vertical line from point P to plane α is called the projection of point P on plane α.

In space, there is only one straight line perpendicular to the known plane at a passing point (on or out of the plane), and its vertical foot is unique, so the projection of the point on the plane is unique and the definition is reasonable.

Three Verticality Theorem: If a straight line in a plane is perpendicular to its projection in the plane, it is perpendicular to the diagonal line passing through the plane.