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I have found this symbol defined as:

    \def\ci{\perp\!\!\!\perp}

on the wikipedia page for Conditional Independence. There's this from the Comprehensive LaTex Symbol List page 106:

Donald Arseneau posted the following \mathpalette-based definition of a probabilistic-independence symbol ⊥⊥ to comp.text.tex in June 2000:

    \newcommand\independent{\protect\mathpalette{\protect\independenT}{\perp}}
    \def\independenT#1#2{\mathrel{\rlap{$#1#2$}\mkern2mu{#1#2}}}

The \independent macro uses \mathpalette to pass the \independenT helper 
macro both the current math style and the \perp symbol. \independenT 
typesets \perp in the current math style, moves two math units to the 
right, and finally typesets a second—overlapping—copy of \perp, again 
in the current math style. \rlap, which enables text overlap, is described 
later on this page.

Frankly, I like the first version better, even though it doesn't typeset properly on my machine.

Are there any better alternatives?

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2 Answers 2

up vote 3 down vote accepted

The usual place at Comprehensive LaTeX Symbol List has the \upmodels symbol in the mnsymbol package, which looks more like the first symbol used in the Wikipedia article. The horizontal distance between the bars is be a bit smaller than the first version, though. See for yourself:

\documentclass{article}
\usepackage{mnsymbol}

\begin{document}

\def\ci{\perp\!\!\!\perp} % from Wikipedia
\newcommand\independent{\protect\mathpalette{\protect\independenT}{\perp}} % symbols-a4, p.106
\def\independenT#1#2{\mathrel{\rlap{$#1#2$}\mkern2mu{#1#2}}} 

$\upmodels$ % mnsymbol
$\independent$
$\ci$
\end{document}
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1  
One thing to note is that the definition of \independenT needs a \mathsurround0pt as in \def\independenT#1#2{\mathrel{\rlap{$#1#2\mathsurround0pt$}\mkern2mu{#1#2}}} Alternatively, \m@th will do the trick. Of course, the best way to do this is to just use the mathtools package and use \mathrlap which takes care of this for you. –  TH. Sep 30 '10 at 23:45
    
A note to future readers: I had to write \usepackage{MnSymbol} in order to make this work. –  Nathaniel Jan 17 at 9:01

In Unicode this is:

⫫ U+2AEB DOUBLE UP TACK

If you have a working unicode-math installation, you can either use the symbol directly, or its alias \Vbar.

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