[Tex/LaTex] Joining curved paths to node anchors in Tikz

tikz-pgf

I am trying to join to two nodes together following the simple flow chart example. I would like to join two blocks with curved arrows that begin and end in the same places on both blocks (or close enough). Instead, I get weird second arrow heads on the original attachment points. I have tried to change the in and out angles but it does not seem to help…

My MWE:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{shapes.arrows,arrows}   
\makeatother
\tikzstyle{block} = [rectangle, draw, fill=blue!50!black!20, 
text width=2cm, text centered, rounded corners, minimum height=2cm, minimum    width=4cm]
\tikzstyle{sblock} = [rectangle, draw, fill=blue!50!black!20, 
text width=2cm, text centered, rounded corners, minimum height=2cm, minimum     width=2cm]
\tikzstyle{line} = [draw, -latex']

\begin{document}
\begin{tikzpicture}[x=1cm,y=1cm]
 % This is the "base case"
   \node [block] at (6,7) (A) {A};  
   \node [sblock] at (1,6) (B) {B};   
   \path [line,thick] (B) edge [ bend right] node {} (A);
   \path [line,thick] (A) edge [bend right] node {} (B);

% This is closer to what I want, but a mess...
   \node [block] at (6,3) (C) {C};  
   \node [sblock] at (1,2) (D) {D};   
   \path [line,thick] (C.west) edge [bend right] node {} (D.east);
   \path [line,thick] (D.east) edge [bend right] node {} (C.west);   

 % No better
   \node [block] at (6,-1) (E) {E};  
   \node [sblock] at (1,-2) (F) {F};   
   \path [line,thick] (E.west) edge [bend right =60] node {} (F.east);
   \path [line,thick] (F.east) edge [bend right=60] node {} (E.west);   
\end{tikzpicture}}
\end{document}

Silly output

As you can see, there is a second arrowhead on the lower block (there is one on the upper block too, it is just obscured). Basically I want the arrows to start on the west/east anchor point.

Best Answer

One way would be to use a to path, e.g. 

\path [line,thick] (E.west) to[out=180,in=60] (F.east);

You can also use edge, but in that case you need to specify the arrow tip for only the edge, not the entire path, e.g.

\path [thick] (C.west) edge [line,bend left] (D.east);

To explain why, I first quote the manual (the second tutorial):

It is now time for Hagen to learn about yet another way of specifying edges: Using the edge path operation. This operation is very similar to the to operation, but there is one important difference: Like a node the edge generated by the edge operation is not part of the main path, but is added only later. This may not seem very important, but it has some nice consequences. For example, every edge can have its own arrow tips and its own color and so on and, still, all the edges can be given on the same path.

While I do not know the details, I guess this is the reason for the double arrow tips. You get one arrow tip from the main path, and another one for the edge, as that is added after the first path.

enter image description here

\documentclass[border=3mm]{standalone}
\usepackage{tikz}
\usetikzlibrary{shapes.arrows,arrows}   

\tikzset{block/.style={rectangle, draw, fill=blue!50!black!20, 
text width=2cm, text centered, rounded corners, minimum height=2cm, minimum    width=4cm},
sblock/.style={rectangle, draw, fill=blue!50!black!20, 
text width=2cm, text centered, rounded corners, minimum height=2cm, minimum     width=2cm},
line/.style={draw, -latex'}}

\begin{document}
\begin{tikzpicture}
   \node [block] at (6,3) (C) {C};  
   \node [sblock] at (1,2) (D) {D};   
   \path [thick] (C.west) edge [line,bend left] (D.east);
   \path [thick] (D.east) edge [line,bend left] (C.west);   


   \node [block] at (6,-1) (E) {E};  
   \node [sblock] at (1,-2) (F) {F};   
   \path [line,thick] (E.west) to[out=180,in=60] (F.east);
   \path [line,thick] (F.east) to[out=0,in=240] (E.west);   
\end{tikzpicture}
\end{document}
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