The principle of duality in the real projective plane is quite a beautiful result.
It's a stunning example of how
![](https://latex.codecogs.com/png.latex?\bg_white \mathbb RP^2 )
"removes the imperfections" from
![](https://latex.codecogs.com/png.latex?\bg_white \mathbb R^2 )
.
(To put it informally, the real projective plane is the union of
![](https://latex.codecogs.com/png.latex?\bg_white \mathbb R^2 )
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
![](https://latex.codecogs.com/png.latex?\bg_white \mathbb RP^2 )
is still true if you replace "point" with "line" and vice versa.
And
![](https://latex.codecogs.com/png.latex?\bg_white \mathbb RP^2 )
has more interesting properties, e.g. the fact that all non-degenerate conic sections are projectively equivalent.