`mathtools` 宏包的 `\prescript` 命令。 如果不想或不能加载 `mathtools` 宏包，可以把相关定义多带带抽取 ```tex \documentclass{article} \usepackage{amsmath} \makeatletter \DeclareRobustCommand{\prescript}[3]{ \mathchoice {\@prescript{#1}{#2}{#3}{\scriptstyle}} {\@prescript{#1}{#2}{#3}{\scriptstyle}} {\@prescript{#1}{#2}{#3}{\scriptscriptstyle}} {\@prescript{#1}{#2}{#3}{\scriptscriptstyle}} } \newcommand{\@prescript}[4]{ \@mathmeasure\z@#4{\prescript@sup{#1}} \@mathmeasure\tw@#4{\prescript@sub{#2}} \ifdim \wd\tw@>\wd\z@ \setbox\z@\hbox to\wd\tw@{\hfil\unhbox\z@} \else \setbox\tw@\hbox to\wd\z@{\hfil\unhbox\tw@} \fi \mathop{} \mathopen{\vphantom{\prescript@arg{#3}}}^{\box\z@}\sb{\box\tw@} \prescript@arg{#3} } \def\prescript@sup{} \def\prescript@sub{} \def\prescript@arg{} \makeatother \begin{document} \[ {}^{4}_{12}\mathbf{C}^{5+}_{2} \quad \prescript{14}{2}{\mathbf{C}}^{5+}_{2} \quad \prescript{4}{12}{\mathbf{C}}^{5+}_{2} \quad \prescript{14}{}{\mathbf{C}}^{5+}_{2} \quad \prescript{}{2}{\mathbf{C}}^{5+}_{2} \] \end{document} ```