# Commutative diagram with parallel arrows in opposite direction

I have a diagram of the following form, written with the package amscd:

\begin{align*}
\begin{CD}
W @>\phi>> X \\
@V{a}V{b}V  @V{c}V{d}V \\
Y @>\phi>> Z \\
\end{CD}
\end{align*}


It want the vertical arrows to be parallel arrows in opposite direction, such that a and c point downwards and b and d point upwards. (For those who understand the terms, it is about an isomorphism between chain complexes.)

To my best knowledge, amscd does not provide these features. Can you show me a piece of code that provides this functionality and is similarly easy to use?

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Do this question and its answers help? –  cgnieder Mar 23 '12 at 14:02

First of all don't use align* for a single object and prefer equation*.

Simple commutative diagrams with the CD environment can have arrows pointing

• right

@>>>

• left

@<<<

• down

@VVV

• up

@AAA


However CD doesn't allow for "double arrows" and you have to go to a more specialized package. For instance xy:

\documentclass{article}
\usepackage{amsmath}
\usepackage[all,cmtip]{xy}
\begin{document}
\begin{equation*}
\xymatrix{
W \ar[r]^{\phi} \ar@<-2pt>[d]_{a} & X \ar@<-2pt>[d]_{c} \\
Y\ar@<-2pt>[u]_{b} \ar[r]^{\phi} & Z \ar@<-2pt>[u]_{d}
}
\end{equation*}
\end{document}


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Thankyou for this answer! It helped me solve the last diagram issue that I had here: –  qu1j0t3 Aug 8 '14 at 2:18

In addition to egregs answer, one could use the package tikz-cd for this:

% arara: pdflatex

\documentclass{article}
\usepackage{tikz-cd}

\begin{document}
$\begin{tikzcd} W \arrow{r}{\phi} \arrow[shift right, swap]{d}{a}& X \arrow[shift right, swap]{d}{c}\\ Y \arrow{r}{\phi} \arrow[shift right, swap]{u}{b} & Z \arrow[shift right, swap]{u}{d} \end{tikzcd}$
\end{document}


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Thanks, I guess that tikz-cd wasn't available at the time of my answer (or it was still too preliminary). –  egreg Oct 9 '14 at 11:10