Ideal polyhedron (https://en.wikipedia.org/wiki/Ideal_polyhedron), a polyhedron in hyperbolic space with all vertices at infinity
Sylvester–Gallai theorem (https://en.wikipedia.org/wiki/Sylvester%E2%80%93Gallai_theorem), that every finite set of points in the Euclidean plane has a line that either passes through all of them or through exactly two of them
Both newly promoted to Good Article status on Wikipedia.
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