A set of concurrent lines in
,
that is, a set of lines passing
through the same point, is a one-dimensional projective space
called a *pencil of lines*. That the space is
one-dimensional is the obvious result of applying the principle of duality:
a set of concurrent lines is the same as a set of collinear points.
We will say no more about a pencil of lines other than to mention that
it exists and that it has several applications in computer vision.