Model for Descriptive Geometry by A. Jullien - Angle between Two Planes with Parallel Horizontal Traces

Jullien, A. · ca 1880

The two planes are APA’ and BQB’, both perpendicular to the vertical plane. As can be seen in the relief, the horizontal images are the parallel lines PA and QB. Each red string represent a line on each plane that is perpendicular to intersection of the plane with the horizontal plane. These lines meet at (m, m’). The locus of all such intersection points is the horizontal line (wire) coming out of the vertical plane. By rotating (m, m’) about segment sm, the image of the angle of intersection of the planes is given by angle sM₁n.