Definition:Below (Solid Geometry)

Definition
Let $a$ and $b$ be points in $3$-dimensional Euclidean space $\R^3$.

Let $P$ be an arbitrary plane embedded in $S$ be distinguished and defined as horizontal.

Then:
 * $a$ is below $b$


 * the height of $a$ $P$ is less than the height of $b$  $P$.
 * the height of $a$ $P$ is less than the height of $b$  $P$.

Also see

 * Definition:Above (Solid Geometry)