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The angle of a line with the PH is the one it makes with its own horizontal projection. It is seen in true size by rotating flat the projecting plane that contains the line and its projection.
Line r and its horizontal projection r′ lie in a plane perpendicular to the PH (horizontal projecting): it contains the verticals joining each point with its projection. The angle we want is in that plane.
With the vertical plane it is the same: compare the line with its vertical projection. The angle with any plane is measured with the perpendicular, using the complement.