Definition:Space Diagonal

Definition
Consider a parallelepiped $P$.

The space diagonals of $P$ are the line segments that can be drawn from each vertex $X$ of $P$ to the vertex $Y$ of $P$ which is not contained in any of the faces of $P$ which contain $X$.


 * SpaceDiagonal.png

In the above diagram, the space diagonals of the parallelepiped $ABCDEFGH$ are $AF$, $BE$, $CH$ and $DG$.