Here is a sampling of how 2D geometry is essentially different from 3D.
| 2D | 3D | |
|---|---|---|
| 1. | There are an infinite number of regular polygons, | but only a finite number of regular polyhedra. |
| 2. | Given two equal-area polygons, one can be dissected into a finite number of pieces, then reassembled into the other. | This is usually not true for pairs of equal-volume polyhedra. |
| 3. | There exists a square that is decomposable into smaller, different, squares. | There is no cube that is decomposable into smaller, different, cubes. |
| 4. | All polygons are decomposable into triangles by adding only interior edges. | Not all polyhedra are decomposable into tetrahedra by adding only interior faces. |
| 5. | For polygons, all such decompositions have the same number of triangles. | Some polyhedra can be decomposed different ways into different numbers of tetrahedra. |
| 6. | Every polygon has every interior point visible from some vertex. | Some polyhedra have interior points not visible from any vertex. |
| 7. | A 2-D Voronoi diagram's complexity is linear. | A 3-D Voronoi diagram's complexity can be quadratic. |
| 8. | Given two convex polygons, there exists an edge of one that separates them. | Given two polyhedra, it is possible that none of their faces separate them. |
| 9. | Rotations commute. | Rotations usually don't commute. |
| 10. | For each edge of a polygon, consider the half-plane of points on the inside side of that edge. Then, the polygon's interior can be expressed as a Boolean expression in those half planes, with each half plane used only once. | Not in 3D. |