# Underbrace under square root sign plain TeX

You could (a) apply a `\smash`

instruction to the `\underbrace{...}_{...}`

part and (b) insert a `\vphantom`

directive *after* the `\sqrt{...}`

part.

```
$$
\sqrt{
\pmatrix{A^{-1} \cr -1}^{\!T}
\smash{\underbrace{\pmatrix{A & b(x) \cr b(x)^T & \Phi(x,x)}}%
_{\textstyle A_{\Phi, X\cup\{x\}}}}
\pmatrix{A^{-1} b(x) \cr -1}
}
\vphantom{\underbrace{\pmatrix{.\cr .}}_{\textstyle A_{\Phi}}}
$$
\bye
```

Observe that I've also dropped some `\displaystyle`

directives and have converted others to `\textstyle`

.

TeX can measure the object:

```
$$
\def\contentsA{\pmatrix{A^{-1} b(x) \cr -1}}
\def\contentsB{\pmatrix{A & b(x) \cr b(x)^T & \Phi(x,x)}}
\setbox0=\hbox{$\displaystyle
\contentsA^{\!T}
{\underbrace{\!\contentsB\!}_{\displaystyle A_{\Phi, X\cup\{x\}}}}
\contentsA
$}
\sqrt{\vphantom{\contentsA^T}\hphantom{\copy0}}
\kern-\wd0
\box0
$$
\bye
```

I made a couple of changes: braces like `{\underbrace{...}_{...}}`

in order to avoid it being considered a math operator with an excess of spacing; for the same reason I introduced a couple of `\!`

tokens. And also one for the exponent in order to place it nearer the top of the parenthesis.