# Create a block matrix with four quadrants

I want to create a block matrix of this form:

But Block A should be a diagonal matrix with 1 on the diagonal, the other blocks should be B, C and D like in the picture. But the size of each block should be equal.

• Welcome to TeX.SX! You can have a look at our starter guide to familiarize yourself further with our format. – karlkoeller Feb 27 '15 at 18:58

\documentclass{article}
$\sbox0{\begin{matrix}1&2&3\\0&1&1\\0&0&1\end{matrix}} % C=\left[ \begin{array}{c|c} \usebox{0}&\makebox[\wd0]{\large B}\\ \hline \vphantom{\usebox{0}}\makebox[\wd0]{\large C}&\makebox[\wd0]{\large D} \end{array} \right]$