The principle of duality in the real projective plane is quite a beautiful result.
It's a stunning example of how
"removes the imperfections" from
.
(To put it informally, the real projective plane is the union of
and a "line at infinity", with the property that parallel lines intersect on the "line at infinity".)
The principle of duality says that any theorem involving points and lines in
is still true if you replace "point" with "line" and vice versa.
And
has more interesting properties, e.g. the fact that all non-degenerate conic sections are projectively equivalent.