# writing bracket under only block vector matrix [duplicate]

I write column vector using \bmatrix then I want to write underbrace and add text in this brace. but I don't how to do this. My code is

\documentclass
[a4paper, 12pt, oneside]{Thesis}
\usepackage{amsmath,amssymb,amstext,amsfonts}
\begin{document}
\begin{align*}
B&=
\begin{pmatrix}
\begin{bmatrix}
1 \\
2 \\
3\\
\underbrace{b_{1}}
\end{bmatrix}
\end{pmatix}
\end{align*}
\end{document}


The bracket is written in column vector but I want to put bracket and b_{1} outside the column vector. Please help me in doing this

## marked as duplicate by TeXnician, Stefan Pinnow, flav, Bobyandbob, Phelype OleinikFeb 8 '18 at 17:34

• Welcome to TeX.SX! To help us help you, please give a compilable minimal working example starting with \documentclass and ending with \end{document}. The above code is not only incomplete, but also erroneous. – Jasper Habicht Feb 8 '18 at 13:14

I guess, what you want is one of these (see also here for more complicated structures):

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{align}
B&=
\underbrace{%
\begin{pmatrix}
\begin{bmatrix}
1 \\
2 \\
3 \\
\end{bmatrix}
\end{pmatrix}%
}_{b_{1}}
\end{align}

\begin{align}
B&=
\begin{pmatrix}
\underbrace{%
\begin{bmatrix}
1 \\
2 \\
3 \\
\end{bmatrix}%
}_{b_{1}}
\end{pmatrix}
\end{align}

\end{document}


• this is exactly what I want. Thank You so much – sam Feb 8 '18 at 14:11