# How can I tikz the equivalence (i.e. $\Leftrightarrow$) as arrow?

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.

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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:

\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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Ohhhhh, so implies is just like < and > (and the direction is automatically correct). Thanks. –  DaveBall aka user750378 Aug 24 '13 at 17:20
Before a confusion arises: >=implies and then <-> works too. The comment with the hypen just means that implies is a arrow head just like latex or to or … –  Qrrbrbirlbel Aug 24 '13 at 18:47