There is a vertical intersection line between faces, and one side has a vertical intersection line, so that line is perpendicular to the other side. As long as this line is vertical, then this line is any straight line on the vertical plane. For example, plane A is perpendicular to plane B, the intersection line is D, and there is a straight line L on plane A. If L is perpendicular to D, then L is a vertical plane B (the vertical plane of a straight line will be perpendicular to any straight line on the plane), then L will be any straight line on the vertical plane B. You can understand it by picking up a pen and a book. But you should note that if two faces intersect but are not perpendicular, then even if a straight line in one face intersects vertically, the straight line is not perpendicular to the other face. The premise that lines intersect vertically and then perpendicular to the surface is that the surface is vertical. You just need to remember that a line perpendicular to the surface will be perpendicular to any straight line in the surface.