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.[br]On the picture below trajectories of points [i]A[/i] and [i]B[/i] in revolution are in [color=#ff0000]red[/color]. Revolved projection rays [i](A)A[sub]1[/sub][/i] and [i](B)B[sub]1[/sub][/i] are perpendicular to top view [i]A[sub]1[/sub]B[sub]1[/sub][/i], distances of points and ground projection plane are in true size and equal to distances of front views [i]A[sub]2[/sub], B[sub]2[/sub][/i] and base line [i]x[/i]. |A[sub]2[/sub]x| =|A[sub]1[/sub](A)|, |B[sub]2[/sub]x| =|B[sub]1[/sub](B)|