Formal Definition
The Moulton plane is an incidence structure, where denotes the set of points, the set of lines and the incidence relation "lies on":
is just a formal symbol for an element . It is used to describe vertical lines, which you may think of as lines with an infinitely large slope.
The incidence relation is defined as follows:
For p = (x, y) ∈ P and g = (m, b) ∈ G we have
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