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

Jullien, A. · ca 1880

The given lines are the black strings: ab’ and dc’. They intersect at (m, m’). Segment ad on the horizontal plane creates a triangle with the given lines as sides. Point n on the horizontal plane is perpendicular to ad through point (m, 0). By rotating the triangle about line ab onto the horizontal plane, point (m, m’) maps to point M₂. Angle aM₂d is the angle between the two given lines.