True size of the segment

The principal lines parallel with projection plane are in true size in front view or top view. To determine true size of oblique line revolve the inclined plane of the line around trace to the position parallel with projection plane. On the picture below trajectories of points A and B in revolution are in red. Revolved projection rays (A)A1 and (B)B1 are perpendicular to top view A1B1, distances of points and ground projection plane are in true size and equal to distances of front views A2, B2 and base line x. |A2x| =|A1(A)|, |B2x| =|B1(B)|