[Tex/LaTex] How to tikz the equivalence (i.e. $\Leftrightarrow$) as arrow

arrowstikz-arrowstikz-pgf

I found in the manual that implies is the right kind of arrow tip, so I tried 

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{arrows}

\begin{document}
\begin{tikzpicture}[>=latex]
  \draw[<->,double equal sign distance,implies] (2,1) -- (3,2);
\end{tikzpicture}
\end{document}

But somehow, implies breaks <->, such that only one end of the line gets an arrow tip.

So how do I get the equivalence as arrow in Tikz?

I searched for another suitable arrow tip (like equivalent) or another suitable parameter instead of <-> to say that both line ends should get the arrow tip, but did not find anything.

Best Answer

implies is the name of a TikZ arrowhead (defined in the arrows TikZ library). It has to be combined with a hyphen (as in my code); it cannot be used as a PGF key on its own (as in your MWE). The following code produces the desired output:

enter image description here

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{arrows}
\begin{document}
\begin{tikzpicture}
\draw[implies-implies,double equal sign distance] (2,1) -- (3,2);
\end{tikzpicture}
\end{document}
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