Model for Descriptive Geometry by A. Jullien - Angle between Two Planes

Jullien, A. · ca 1880

The given planes are APA’ and BQB’. The vertical and horizontal projections of the planes meet at the points d’ and c on the vertical and horizontal planes respectively, with line of intersection of the two planes cd’, depicted by the red sting. Point n is the foot of the perpendicular from the line to the horizontal plane. The red string that runs left to right indicates the plane perpendicular to line cd’ with segments on the given planes. By rotating this perpendicular plane down to the horizontal plane, the angle between the two given planes is the angle tM₂s.