$\begin{CD} A @>a>> B\\ @VVbV @VVcV\\ C @>d>> D \end{CD}$
$\begin{CD} A @>a>b> B\\ @VlVrV @AlArA\\ C @<a<b< D \end{CD}$
$\begin{CD} A @<<< B @>>> C\\ @. @| @AAA\\ @. D @= E \end{CD}$
$\begin{CD} A @>>> B @>{\text{very long label}}>> C \\ @VVV @VVV @VVV \\ D @>>> E @>{\rlap{\scriptstyle{\ \ \ \text{shorter}}}\phantom{\text{very long label}}}>> F \end{CD}$