Editing Conical surface (section)

==Definitions==
A (''general'') conical surface is the unbounded surface formed by the union of all the straight lines that pass through a fixed point &mdash; the ''apex'' or ''vertex'' &mdash; and any point of some fixed [[space curve]] &mdash; the ''directrix'' &mdash; that does not contain the apex.  Each of those lines is called a ''generatrix'' of the surface. The directrix is often taken as a [[plane curve]], in a plane not containing the apex, but this is not a requirement.<ref>{{citation|title=The Theory of Engineering Drawing|first=Alphonse A.|last=Adler|publisher=D. Van Nostrand|year=1912|contribution=1003. Conical surface|contribution-url=https://archive.org/details/cu31924003943481/page/n185|page=166}}</ref>

In general, a conical surface consists of two congruent unbounded halves joined by the apex.  Each half is called a '''nappe''', and is the union of all the [[Line (mathematics)#Ray|ray]]s that start at the apex and pass through a point of some fixed space curve.<ref name=msg>{{citation|title=Modern Solid Geometry, Graded Course, Books 6-9|first1=Webster|last1=Wells|first2=Walter Wilson|last2=Hart|publisher=D. C. Heath|year=1927|pages=400–401|url=https://books.google.com/books?id=vXENAQAAIAAJ&pg=PA400}}</ref> Sometimes the term "conical surface" is used to mean just one nappe.<ref>{{citation|title=Solid Geometry|first=George C.|last=Shutts|publisher=Atkinson, Mentzer|year=1913|contribution=640. Conical surface|page=410|contribution-url=https://books.google.com/books?id=9zAAAAAAYAAJ&pg=PA410}}</ref>