Shouldn't "horizontal" and "vertical" be exchanged in perpendicular coordinates?

[knuesel]

The documentation for perpendicular coordinates says

horizontal: The coordinate through which the horizontal line passes.
vertical: The coordinate through which the vertical line passes.

You can use the implicit syntax of (horizontal, "-|", vertical) or (vertical, "|-", horizontal).

while the actual behavior of the keywords seems like the other way round. Look at the example in the documentation:

line("p1.south-east", (horizontal: (), vertical: "xline.end"))
line("p2.south-east", ((), "|-", "xline.end")) // Short form

In the first line it's actually the horizointal line that goes through `xline.end``.

The short form looks right: - indicates a horizontal line going through xline.end.

[knuesel]

I guess the root of the problem is that the keyword names horizontal and vertical are ambiguous, they can be understood as horizontal/vertical line (as documented) or horizontal/vertical coordinate (as implemented). Tikz uses the keys horizontal line through and vertical line through which remove the ambiguity but are very verbose.

[johannes-wolf]

Maybe something like take-x/take-y/take-z is more explanatory?

[knuesel]

Would be much clearer but is it always the case that x and y refer respectively to the horizontal and vertical coordinates in cetz?

[knuesel]

I see now that taking the x or y value is exactly what this coordinate system does (even when those don't correspond to horizontal or vertical directions) so take-x/take-y/take-z look like great names.