Question: How could I proceed to make an "equal" arrow in a diagram in tikz? To be clearer, what I would like to obtain is the equivalent of the following command in xy:

> 
> \ar@{=}[direction]
> 
> 
> 

Two examples of what I want (done in tikz):

![](https://i.stack.imgur.com/DIpDm.png)

and

![](https://i.stack.imgur.com/JKYpk.png)

Thank you for your help.

Answer: 
You can use `double equal sign distance` in your `\draw` options to set the distance between the double lines to match that of an equal sign.

The double lines work with any path specification, including things like `bend left` or `to [out=45]`:

![](https://i.stack.imgur.com/VsNq7.png)

```
\documentclass[border=5mm]{standalone}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
\draw [double equal sign distance] (0,0) to [out=45, in=180] (3,0) node [anchor=mid west] {$= A$};
\end{tikzpicture}
\end{document}

```
The great [`tikz-cd`](http://www.ctan.org/pkg/tikz-cd) package ([tikz-cd](/questions/tagged/tikz-cd "show questions tagged 'tikz-cd'"), [manual](http://texdoc.net/texmf-dist/doc/latex/tikz-cd/tikz-cd-doc.pdf)) includes the option `equal` for arrows.

Code
----

```
\documentclass[tikz,convert=false]{standalone}
\usepackage{tikz-cd}
\usetikzlibrary{arrows}
\tikzset{commutative diagrams/.cd,arrow style=tikz,diagrams={>=latex'}}
\begin{document}
\begin{tikzcd}[swap,bend angle=45]
A \dar{f} \rar{a} \ar[bend left,equal]{rr}
  & X \dar{g} \rar{r}
      & A \dar[swap]{f} \\
B \rar[swap]{i}   \ar[bend right,equal]{rr}
      & Y \rar[swap]{\beta}
          & B
\end{tikzcd}
\end{document}

```

Output
------

![enter image description here](https://i.stack.imgur.com/C21Vs.png)