# In XY-pic, two parallel arrows passing under a shape are not parallel when they are separated by distance

I want to draw a commutative diagram with xy-pic package similar to the figure shown below. The two arrows from C to D and vice versa should pass under the rectangle ABEF, be separated by a distance, and be horizontal. The problem is that I can not make the segments of these two arrows to be horizontal, rather they are tilted as shown. The figure is generated by this code:

\begin{equation*}
\xymatrix@!=3.5pc{
& A \ar[dd] \ar[r] & B \ar[dd] & \\
C \ar@<2ex>'[r]'[rr][rrr] &&& D  \ar@{->}@<2ex>'[l]'[ll][lll] \\
& E \ar[r] & F &
}
\end{equation*}


When the two arrows are placed exactly on each other (that is, zero distance) both are horizontal and there is no problem.

However, when I make a distance between them, they will not stay horizontal and each segment of arrow tilts. In above code I used 2ex distance between arrows to exaggerate the problem. Though, I originally intend to use smaller distances. How can I fix this? Thanks.

You can also get crossing arrows by putting a small gap in the arrow to be broken by breaking the arrow with the special label \hole. To see a discussion and examples, take a look at the AMS-LaTeX primer "Getting up and running with AMS-LaTeX"amshelp.pdf, at https://ctan.org/tex-archive/info/amslatex/primer?lang=en, section 8.6 (More crossing arrows). For your particular question:

\documentclass{amsart}
\usepackage[all,cmtip]{xy}

\begin{document}
\begin{equation*}
\xymatrix@!=3.5pc{
& A \ar[dd] \ar[r] & B \ar[dd] & \\
C
\ar@<2ex>|!{[ru];[rd]}{\hole}|!{[rru];[rrd]}{\hole}[rrr]
&&& D
\ar@<2ex>|!{[lu];[ld]}{\hole}|!{[llu];[lld]}{\hole}[lll]\\
& E \ar[r] & F &
}
\end{equation*}
\end{document}

• Amazing! (I would have thought this is too tough for xy, so +1.)
– user121799
May 31, 2019 at 3:50
• @marmot Thanks; I do admit that the tikz-cd approach is overall cleaner and more intuitive. May 31, 2019 at 3:57
• I guess it is always a matter what one is more familiar with. I think your solution is simple and good, and answers the question, which is on xy.
– user121799
May 31, 2019 at 3:59

Just for fun: a tikz-cd realization.

\documentclass{article}
\usepackage{tikz-cd}
\begin{document}
\begin{tikzcd}[column sep=2em,row sep=2em]
& A \arrow[r]  & B  & \\
C \arrow[rrr,yshift=0.65ex]& & & \arrow[lll,yshift=-0.65ex] D\\
& E \arrow[r] \arrow[uu,<-,crossing over] & F \arrow[uu,<-,crossing over]& \\
\end{tikzcd}
\end{document} 