The routine returns the name of the vertical axis used to define obj, It returns either a name or FAIL.

The environment variables _EnvHorizontalName and _EnvVerticalName can be assigned names that are used as the names of the horizontal and vertical axes, respectively.

The command with(geometry,VerticalName) allows the use of the abbreviated form of this command.

$\mathrm{with}\left(\mathrm{geometry}\right)\:$

$\mathrm{point}\left(A\,0\,0\right),\mathrm{point}\left(B\,1\,1\right)\:$

$\mathrm{line}\left(l\,\left[A\,B\right]\right)$

$l$

$\mathrm{detail}\left(l\right)$

assume that the names of the horizontal and vertical axes are _x and _y, respectively

$\begin{array}{ll}\text{name of the object}& l\\ \text{form of the object}& \mathrm{line2d}\\ \text{equation of the line}& -\mathrm{\_x}+\mathrm{\_y}=0\end{array}$

$\mathrm{VerticalName}\left(l\right)$

$\mathrm{FAIL}$

$\mathrm{\_EnvHorizontalName}≔a\:$$\mathrm{\_EnvVerticalName}≔b\:$

$\mathrm{Equation}\left(l\right)$

$-a+b=0$

$\mathrm{VerticalName}\left(l\right)$

$b$

$\mathrm{detail}\left(l\right)$

$\begin{array}{ll}\text{name of the object}& l\\ \text{form of the object}& \mathrm{line2d}\\ \text{equation of the line}& -a+b=0\end{array}$