title: Ridge (differential geometry)
text: # Ridge (differential geometry)
  
  In differential geometry, a smooth surface in three dimensions has a ridge point when a line of curvature has a local maximum or minimum of principal curvature. The set of ridge points form curves on the surface called ridges.
  
  The ridges of a given surface fall into two families, typically designated red and blue, depending on which of the two principal curvatures has an extremum.
  
  At umbilical points the colour of a ridge will change from red to blue. There are two main cases: one has three ridge lines passing through the umbilic, and the other has one line passing through it.
  
  Ridge lines correspond to cuspidal edges on the focal surface.
  
  