Alignment base on two reference nodes

Question

I am new to tikz. If I have two nodes [A] and [B] as:

[A]

       [B]

How do I align [C] so that it is vertically below [A] and horizontally left of [B] such that:

[A]

[C]    [B]

without resorting to absolute positioning?

Answer 1

\documentclass[11pt]{scrartcl}
\usepackage{tikz}

\begin{document}
\begin{tikzpicture}
\node (a) at (0,0){[A]};
\node (b) at (5,-4){[B]};  
\node (c) at (a|-b){[C]};
\end{tikzpicture}
\end{document} 

Answer 2

You can use relative positioning (§ 3.8; TIKZ & PGF Manual, v. 2.10).

 \documentclass{article}
 \usepackage{tikz}
 \begin{document}
 \begin{figure}
 \centering
 \begin{tikzpicture}[auto]
 \node [draw] (a) {A};
 \node [draw, below of=a] (c) {C};
 \node [draw, right of=c] (b) {B};
 \end{tikzpicture}
 \end{figure}
 \end{document}