# (easy?) Matrix with wide blocks

I'd like to create a matrix like this:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
$$\begin{bmatrix} a & b' \\ b & D \end{bmatrix}$$
\end{document}


but I'd like to indicate that the size of vector b and put it like on two fields, horizontally or vertically. The same with matrix D which I'd like to put like a 2×2 matrix with a name in the center.

• Welcome to TeX.SX! Could you please complete your code in order to make a minimal working example (MWE) that inlcudes the relevant packages as well as the documentclass? Could you please also include a sketch of the output you'd like to achieve? Mar 2 '19 at 14:26
• Now it is good that you already provided an MWE, so I upvoted your question, but I can't still get it. Please add a sketch showing what you want.
– user156344
Mar 2 '19 at 14:36
• Thank you for helping me with my first post. I have some experience at other community where it would automatically compile. Polish one. Mar 2 '19 at 14:40
• You remember that $$...$$ is TeX primitive syntax. Mar 2 '19 at 16:56
• @Sebastiano what does it mean? Mar 2 '19 at 17:24

\documentclass{article}
$\sbox0{\begin{matrix} D & D\end{matrix}} \left[\begin{array}{c|c} a & \mathmakebox[\wd0]{b'} \\ \hline b & \vphantom{\begin{matrix} D \\ D \end{matrix}} D \end{array}\right]$