\begin{definition}[Definition:Projection (Analytic Geometry)/Plane]
Let $M$ and $N$ be distinct lines in the plane.
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The '''projection on $M$ along $N$''' is the mapping $\pr_{M, N}$ such that:
:$\forall x \in \R^2: \map {\pr_{M, N} } x =$ the intersection of $M$ with the line through $x$ parallel to $N$.
\end{definition}