More generally, a pencil is the special case of a linear system of divisors in which the parameter space is a projective line. Typical pencils of curves in the projective plane, for example, are written as
- λC + μC′ = 0
- C = 0, C′ = 0
are plane curves.
A pencil of planes, the family of planes through a given straight line, is sometimes referred to as a fan.
See also 
- Romuald Ireneus 'Scibor-Marchocki. "Perspective". Elementary-Geometry Trigonometry.
General references 
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