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\strut

Synopsis:

\strut

Ensure that the current line has height at least 0.7\baselineskip and depth at least 0.3\baselineskip. Essentially, LaTeX inserts into the line a rectangle having zero width, \rule[-0.3\baselineskip]{0pt}{\baselineskip} (see \rule). The \baselineskip changes with the current font and fontsize.

In this example the \strut keeps the box inside the frame from having zero height.

\setlength{\fboxsep}{0pt}\framebox[2in]{\strut}

This example has four lists. In the first there is a much bigger gap between items 2 and 3 than there is between items 1 and 2. The second list fixes that with a \strut at the end of its first item’s second line.

\setlength{\fboxsep}{0pt}
\noindent\begin{minipage}[t]{0.2\linewidth}
\begin{enumerate}
  \item \parbox[t]{15pt}{test \\ test}
  \item test
  \item test
\end{enumerate}
\end{minipage}%
\begin{minipage}[t]{0.2\linewidth}
\begin{enumerate}
  \item \parbox[t]{15pt}{test \\ test\strut}
  \item test
  \item test
\end{enumerate}
\end{minipage}%
\begin{minipage}[t]{0.2\linewidth}
\begin{enumerate}
  \item \fbox{\parbox[t]{15pt}{test \\ test}}
  \item \fbox{test}
  \item \fbox{test}
\end{enumerate}
\end{minipage}%
\begin{minipage}[t]{0.2\linewidth}
\begin{enumerate}
  \item \fbox{\parbox[t]{15pt}{test \\ test\strut}}
  \item \fbox{test}
  \item \fbox{test}
\end{enumerate}
\end{minipage}%

The final two lists use \fbox to show what’s happening. The third list’s \parbox goes only to the bottom of its second ‘test’, which happens not have any characters that descend below the baseline. The fourth list adds the strut that gives the needed extra below-baseline space.

The \strut command is often useful in graphics, such as in TikZ or Asymptote. For instance, you may have a command such as \graphnode{node-name} that fits a circle around node-name. However, unless you are careful the node-name’s ‘x’ and ‘y’ will produce different-diameter circles because the characters are different sizes. A careful \graphnode might insert a \strut, then node-name, and then draw the circle.

The general approach of using a zero width \rule is useful in many circumstances. In this table, the zero-width rule keeps the top of the first integral from hitting the \hline. Similarly, the second rule keeps the second integral from hitting the first.

\begin{tabular}{rl}
  \textsc{Integral}   &\textsc{Value}           \\
  \hline
  $\int_0^x t\, dt$   &$x^2/2$  \rule{0em}{2.5ex} \\  
  $\int_0^x t^2\, dt$ &$x^3/3$  \rule{0em}{2.5ex}
\end{tabular}

(Although the line-ending double backslash command has an available optional argument to put in more vertical room, that won’t work here. Changing the first double backslash to something like \\[2.5ex] will put the room between the header line and the \hline, and the integral would still hit the line.)