Editing Conical surface (section)

==Related surface==
Conical surfaces are [[ruled surface]]s, surfaces that have a straight line through each of their points.<ref>{{citation|title=Encyclopedic Dictionary of Mathematics, Vol. I: A–N|edition=2nd|publisher=MIT Press|editor-first=Kiyosi|editor-last=Ito|author=Mathematical Society of Japan|year=1993|page=419|url=https://books.google.com/books?id=WHjO9K6xEm4C&pg=PA419}}</ref> Patches of conical surfaces that avoid the apex are special cases of [[developable surface]]s, surfaces that can be unfolded to a flat plane without stretching. When the directrix has the property that the angle it subtends from the apex is exactly <math>2\pi</math>, then each nappe of the conical surface, including the apex, is a developable surface.<ref>{{citation|title=Elasticity and Geometry: From Hair Curls to the Non-linear Response of Shells|first1=Basile|last1=Audoly|first2=Yves|last2=Pomeau|publisher=Oxford University Press|year=2010|isbn=9780198506256|pages=326–327|url=https://books.google.com/books?id=FMQRDAAAQBAJ&pg=PA326}}</ref>

A [[cylindrical surface]] can be viewed as a [[limiting case (mathematics)|limiting case]] of a conical surface whose apex is moved off to infinity in a particular direction. Indeed, in [[projective geometry]] a cylindrical surface is just a special case of a conical surface.<ref>{{citation|title=Descriptive Geometry|first1=F. E.|last1=Giesecke|first2=A.|last2=Mitchell|publisher=Von Boeckmann–Jones Company|year=1916|page=66|url=https://books.google.com/books?id=sCc7AQAAMAAJ&pg=PA66}}</ref>