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Over- and Underlining

LaTeX provides commands for making overlines or underlines, or putting braces over or under some material.

\underline{text}

Underline text. Works inside math mode, and outside. The line is always completely below the text, taking account of descenders, so in \(\underline{y}\) the line is lower than in \(\underline{x}\). This command is fragile (see \protect).

Note that the package ulem does text mode underlining and allows line breaking as well as a number of other features. See the documentation on CTAN. See also \hrulefill & \dotfill for producing a line, for such things as a signature.

\overline{text}

Put a horizontal line over text. Works inside math mode, and outside. For example, \overline{x+y}. Note that this differs from the command \bar (see Math accents).

\underbrace{math}

Put a brace under math. For example, this (1-\underbrace{1/2)+(1/2}-1/3) emphasizes the telescoping part. Attach text to the brace by using 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, as with \overbrace{x+x+\cdots+x}^{\mbox{\(k\) times}}. See also \underbrace.

The package mathtools adds an over- and underbrace, as well as some improvements on the braces. See the documentation on CTAN.