%%
%% The LaTeX Companion, 3ed
%%
%% Example 11-4-2 on page II-163 in "amsmath, mathtools, extarrows --- Decorated arrows".
%%
%% Copyright (C) 2022 Frank Mittelbach
%%
%% It may be distributed and/or modified under the conditions
%% of the LaTeX Project Public License, either version 1.3c
%% of this license or (at your option) any later version.
%%
%% See https://www.latex-project.org/lppl.txt for details.
%%

\documentclass{tlc3examargin}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{153.0pt}

  % Shorten the vertical size of the example ...
  \AtBeginDocument{%
    \setlength\abovedisplayskip{.8\abovedisplayskip}%
    \setlength\belowdisplayskip{.8\belowdisplayskip}%
  }

%StartShownPreambleCommands
\usepackage{mathtools}
%StopShownPreambleCommands

\begin{document}
\begin{gather}
  A \xLeftarrow{\ x} B \xRightarrow[y\ ]{}
                   C \xLeftrightarrow[y]{\ x\ } D    \\
  A \xhookleftarrow{x} B \xhookrightarrow[bbb]{a} C  \\
  \xleftharpoondown{} \xleftharpoonup{}
  \xleftrightharpoons{xxx} \xrightleftharpoons[yy]{}
  \xrightharpoondown{} \xrightharpoonup{}            \\
  \xmapsto{x>0} A \xleftrightarrow[\text{letters}]{} Z
\end{gather}
\end{document}