Plane
A flat horizontal surface, with grid subdivisions.

The Plane is a surface with zero thickness, created lying on the XZ plane with the face pointing up. With no rotation at all, it already works as a floor.
Properties
- Width and Depth: Measurements of the surface, in meters, on the X and Z axes.
- Width Seg. and Depth Seg.: Number of grid divisions in each direction. They only matter when something is going to deform the surface, such as a displacement modifier or sculpting, because a single face has nowhere to bend.
One side only
The plane has a face on one side only and disappears when seen from below, because faces turned away are not drawn. For a surface visible from both sides, duplicate it and flip the copy, or use the solidify modifier to give it thickness.