Topic summary
Horizontal plane
Extracted from the Wikipedia article Vertical and horizontal.
In three dimensions
In the three-dimensional case, the situation is more complicated as now one has horizontal and vertical planes in addition to horizontal and vertical lines. Consider a point P and designate a orientation through P as vertical. A plane which contains P and is normal to the designated orientation is the horizontal plane at P. Any plane going through P, normal to the horizontal plane is a vertical plane at P. Through any point P, there is one and only one horizontal plane but a multiplicity of vertical planes. This is a new feature that emerges in three dimensions. The symmetry that exists in the two-dimensional case no longer holds.