From Wiki Dual polyhedron
For example the smplices:
3-simplex (tetrahedron) is self-dual
2-simplex (triangle) is self-dual
1-simplex (line segment) ??
3-cube (cube) dual is octahedron
2-cube (square) dual is square, self-dual
1-cube (line segment) ??
In the above link you can see it says all regular simplices are self-dual, but I can't see how a line segment is self-dual. Shouldn't its dual have 1 vertex and 2 edges? How would you draw that? Is there a rigorous definition of duality in this geometric sense?