\begin{definition}[Definition:Axis/Z-Axis/Right-Hand Rule]
The usual convention for the orientation of the $z$-axis is that of the right-hand rule:
Let the coordinate axes be oriented as follows:
:Let the $x$-axis increase from '''West''' to '''East'''.
:Let the $y$-axis increase from '''South''' to '''North'''.
Then the $z$-axis increases from '''below''' to '''above'''.
If the $x$-axis and $y$-axis are aligned with a piece of paper or a screen aligned perpendicular to the line of sight, this translates into the following orientation:
:Let the $x$-axis increase from '''left''' to '''right'''.
:Let the $y$-axis increase from '''bottom''' to '''top'''.
Then the $z$-axis increases from '''behind''' to '''in front''' (that is, from '''further away''' to '''closer in''').
\end{definition}