Simon's Law:
Everything its topput together falls apart sooner or later

Concept 3.1 : What is a frustum?
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Idea:
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The frustum is a truncated pyramid whose top has been 
cut off by a plane parallel to its base.  The apex of
this pyramid is at the origin.
  
The four sides of the frustum, its top and its base 
define the six clipping planes of the viewing volume.
Objects or parts of objects outside these planes
are clipped from the final image.

The perspective viewing volume is specified using the 
following function :
glFrustum(left, right, bottom, top, near, far);
This creates a matrix that accomplishes perspective
projection and multiplies the current projection matrix
(typically the identity matrix) by it.  The frustum's
viewing volume is defined by the parameters : (left, 
bottom, -near) and (right, top -near) specify the (x,y,z)
coordinates of the lower-left and upper-right corners
of the near clipping plane; near and far give the 
distances from the viewpoint to the near and far clipping
planes.  Both near and far should be positive.