Math mode: square bracket strange behaviour

Question

Can you see as the content is not aligned with square brackets?

\documentclass[12pt]{article}
\usepackage{amsmath}
\begin{document}
\[
  \left[ \dfrac{1}{\dfrac{\mathrm{L}^3}{\mathrm{t}}} \right]
\]
\end{document}

Content should be displayed higher. Maybe a \dfrac bug? Or maybe I should fine an alternatice to \left[ and \right]?

Answer 1

That's the expected behavior: the brackets are symmetric with respect to the formula axis, which is the imaginary line where the main fraction line sits on.

I guess that this is some step during dimensional analysis; suppose you follow the advice by Bernard, but put it in context:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\[
\begin{bmatrix}
\dfrac{1}{\dfrac{\mathrm{L}^3}{\mathrm{t}}}
\end{bmatrix}
=
\left[\frac{\mathrm{t}}{\mathrm{L}^3}\right]
\]

\end{document}

Your readers will have a very hard time in interpreting the meaning of such a formula. Which one is the main fraction line? The top one is just slightly longer than the (secondary) bottom one and L³ lies almost exactly at the same level as the fraction line in the right-hand side.

Take into account that mistaking the main fraction line would lead to very wrong interpretation.

Making the main fraction line longer doesn't really solve the problem:

\[
\begin{bmatrix}
\dfrac{\;\;1\;\;}{\dfrac{\mathrm{L}^3}{\mathrm{t}}}
\end{bmatrix}
=
\left[\frac{\mathrm{t}}{\mathrm{L}^3}\right]
\]

would produce

which is as unreadable.

Solution: use exponents, as is commonly done in dimensional analysis:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\[
\left[\frac{1}{\mathrm{L}^3\mathrm{t}^{-1}}\right]
=
[\mathrm{L}^{-3}\mathrm{t}]
\]

\end{document}

Answer 2

You obtain a better looking formula with bmatrix:

\documentclass[12pt]{article}
\usepackage{amsmath}

\begin{document}

\[
  \begin{bmatrix}\dfrac{1}{\dfrac{\mathrm{L}^3}{\mathrm{t}}} \end{bmatrix}
\]

\end{document}