ENTITY allows you to define a plane using a selected line,
arc, conic, or 2-D spline. A line entity defines the plane
as being normal to the screen, at the same inclination as
the line projected onto the screen. An arc or spline 
entity defines the plane in the view of definition and at 
the depth of the selected entity.

Select the desired line, arc, conic, or 2-D spline.
