LaTeX provides commands for putting lines, braces, and arrows over or under math material.
\underline{math} ¶Underline math. For example: \underline{x+y}.
The line is always completely below the text, taking account of
descenders, so in \(\underline{y}\) the line is lower than in
\(\underline{x}\). As of approximately 2019, this command
and others in this section are robust; before that, they were fragile
(see \protect).
The package ulem (https://ctan.org/pkg/uelem) does
text mode underlining and allows line breaking as well as a number of
other features. See also \hrulefill & \dotfill for
producing a line for such things as a signature or placeholder.
\overline{math} ¶Put a horizontal line over math. For example: \overline{x+y}.
This differs from the accent command \bar (see Math accents).
\underbrace{math} ¶Put a brace under math. For example:
(1-\underbrace{1/2)+(1/2}-1/3).
You can attach text to the brace as a subscript (_) or
superscript (^) as here:
\begin{displaymath}
1+1/2+\underbrace{1/3+1/4}_{>1/2}+
\underbrace{1/5+1/6+1/7+1/8}_{>1/2}+\cdots
\end{displaymath}
The superscript appears on top of the expression, and so can look unconnected to the underbrace.
\overbrace{math} ¶Put a brace over math. For example:
\overbrace{x+x+\cdots+x}^{\mbox{\(k\) times}}.
\overrightarrow{math} ¶Put a right arrow over math. For example:
\overrightarrow{x+y}.
\overleftarrow{math} ¶Put a left arrow over math. For example:
\overleftarrow{a+b}.
The package mathtools (https://ctan.org/pkg/mathtools)
adds an over- and underbracket, as well as some improvements on the
braces.