TikZ: What EXACTLY does the the |- notation for arrows do?

Question

I am currently trying to work out how the perpendicular arrows work, and I fail.

Taking:

\draw [arrow] (node1) -- node[sloped, above] {Caption} (node1 |- node2.north west);

I suspected that the notation meant:
Draw an arrow from node 1, to the north-west corner of node 2.
Make the arrow perpendicular to node 1.

Further attempts to draw arrows told me: No, that's not it. I have sadly failed to figure out what exactly the notation is supposed to be doing though! Is there any "explanation for dummies" that will lead me to understand what exactly

node1 |-

Is supposed to do?

Thanks!

Answer 1

There are two different places to use -|/|-:

1. In a coordinate specification.

   This is what you have used, the general form is

   (a -| b)
   

   where a and b are named nodes or coordinates. This means the coordinate that is at the y-coordinate of a, and x-coordinate of b. Similarly, (a |- b) has the x-coordinate of a and y-coordinate of b.

   For example, the following code draws a horizontal arrow from a at (0,0) to (1,0).

   \documentclass[border=10pt]{standalone}
   \usepackage{tikz}
   \begin{document}
   \begin{tikzpicture}
   \coordinate (a) at (0,0);
   \coordinate (b) at (1,1);
   
   \draw [->] (a) -- (a -| b);
   \end{tikzpicture}
   \end{document}
   

   You can also use this as

   \coordinate (c) at (a -| b);
   

   and then \draw [->] (a) -- (c); does the same as the above.

2. As a path specification.

   This is used between two coordinates, in place of --.

   With \draw [->] (a) -| (b); the arrow goes horizontally from a, then vertically up to b. (And with |- it would be vertically first, then horizontally.)

   

   \documentclass[border=10pt]{standalone}
   \usepackage{tikz}
   \begin{document}
   \begin{tikzpicture}
   \coordinate (a) at (0,0);
   \coordinate (b) at (1,1);
   
   \draw [->] (a) -| (b);
   \end{tikzpicture}
   \end{document}
   

Answer 2

If a is a coordinate/node at position (a_x, a_y) and
b is at (b_x, b_y) then

a |- b := (a_x, b_y) 

a -| b := (b_x, a_y) 

They're just shorthands for combining the x/y coordinates of two points.