: Search out info from Barry, Rabl, or Kilgore.
: Depending on the design - even relatively simple ones, there can be
: many 'apexes' of many cones. In simple terms for an existing
: design, one takes the 2 views (side and top, say) and draw a
: ruling line on one view extending outward and that line's
: projection on the other view outward to the point that their
: horizontal coordinate is the same. If an adjacent ruling line
: can intersect that horizontal coordinate and still be true to
: the 2 views, then move on and see if the next ruling line will
: also use that same apex. If not, try another apex for that 3rd
: line. If it works for the full length of the shape, you're in
: luck- if not, try other ruling lines and apexes. The notion
: being used is that 2 closely adjacent lines can only be on the
: same plane (ie developable surface) if they are either exactly
: parallel or that they intersect.
: I have not explained it very well, the big boys mentioned will do
: better.
: -mick

Mick, I downloaded Barry's excellent paper some time ago and found it very helpful. I have just found a download by Rable and will search for material by Kilgore. It is not an easy concept to explain in words only.