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I was writing some macros for drawing timelines in Tikz and thought that it might be good to name the internal help coordinates with an @ in it. The idea is that in this way it will not conflict with the coordinates of a potential user of those macros (not that I will be able to produce a package because of my recurrent coding problems).

However, I was surprised that I can use an @ in the name of a coordinate without placing it within \makeatletter and \makatother:

\begin{tikzpicture}
  \draw (0,0)--(2,0) coordinate (A@1);
  \draw (1,1)--(A@1);
\end{tikzpicture}

My question is: why does it work?

Then, if the @ can be used freely, the help coordinates will not be protected against an unintentional use.

Raoul Kessels
  • 2,693
  • The fact that one can use @ in coordinate names is well known and has used e.g. in https://tex.stackexchange.com/a/316050/121799. The idea is still that using these makes it less likely that "random users" accidentally use the same symbols. Same is true for underscores, e.g. \begin{tikzpicture} \draw (0,0) --(2,0) coordinate (A@1) (4,0) coordinate (X_Y); \draw (1,1)--(A@1) -- (X_Y); \end{tikzpicture}. –  Apr 27 '19 at 19:23

1 Answers1

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the distinction between letters and non letters (catcode 11 or not) is just when parsing after the escape character (usually \) so \foo@bar normally parses as the 5 tokens \foo, @, b, a, r. But the foo@bar csname can be accessed without changing the catcode of @ by using \csname foo@bar\endcsname for example.

You see a similar thing with environment names where \begin{tabular*} access the command with name tabular* even though \tabular* would not access that but the two tokens \tabular and * unless you change the catcode of * to 11.

David Carlisle
  • 757,742
  • So the coordinate names are converted to commands through \csname? – Raoul Kessels Apr 27 '19 at 19:45
  • @RaoulKessels most likely, but whatever tikz does with them, the catcode status of @ has no effect on anything but the parsing of \abc@xyz so it will not affect the use you show with coordinate (A@1) (well other than explicit testing with \ifcat ) – David Carlisle Apr 27 '19 at 19:48