positioning a child node between two parents

Question

\begin{tikzpicture}[node distance=1.5cm]
 \node[state] (A) {A};
 \node[state] (C) [right of=A] {C};
 \node[state] (B) [below in the middle of=A and C] {B}
\end{tikzpicture}

Something like this:

 A C
 \/
 B 

I couldn't figure out how to do it. I believe there is Tree extension but I want to write the graph without using it.

Answer 1

1. First draw the node B. Then use above left and above right to draw the nodes A and B respectively. OR
2. Use xshift and push the node B by half of the length between nodes. (You know the node distance). But in this method the minimum distance between the node A (or C) and node B is not 1.5cm but greater. OR
3. First draw A and then B and then C.

And in cases 1 and 3 the nodes A and c are not separated by 1.5cm :-(.

Code:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{positioning}

\begin{document}
\begin{tikzpicture}[node distance=1.5cm]
 \node at (0,0) (B) {B};
 \node (C) [above right of=B] {C};
 \node (A) [above left of=B] {A};
\end{tikzpicture}
\begin{tikzpicture}[node distance=1.5cm]
 \node at (0,0) (A) {A};
 \node (C) [right of= A] {C};
 \node (B) [below of=A,xshift=.75cm] {B};
\end{tikzpicture}
\begin{tikzpicture}[node distance=1.5cm]
 \node at (0,0) (A) {A};
 \node (B) [below right of= A] {B};
 \node (B) [above right of=B] {C};
\end{tikzpicture}
\end{document}

Answer 2

You can use tikz's calc library to compute the mid point and place the node below that:

Code:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{calc}
\begin{document}
\begin{tikzpicture}[node distance=1.5cm]
 \node (A) {A};
 \node (C) [right of=A] {C};
 \coordinate (Middle) at ($(A)!0.5!(C)$);
 \node (B) [below of=Middle] {B};
\end{tikzpicture}
\end{document}

Answer 3

Here is a solution using pos=.5 to find the middle point of path A to C and positioning TikZ library to put the B node 1cm below this middle point:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{positioning}
\begin{document}
\begin{tikzpicture}[node distance=1.5cm]
 \node (A) {A};
 \node (C) [right of=A] {C};
 \path (A) -- (C) node[pos=.5,below=1cm] (B) {B};
\end{tikzpicture}
\end{document}