Examples from the TeXbook
(Chapter 18, Commutative Diagram)
TeXbook, Exercise 18.46
\matrix{
&&&&&&0\cr
&&&&&&\mapdown{}\cr
\rule 0pt 1.1em .6em
0&\mapright{}&{\cal O}_C&\mapright\iota&\cal E&
\mapright\rho&\cal L&\mapright{}&0\cr
&&\Big\Vert&&\mapdown\phi&&\mapdown\psi\cr
\rule 0pt 1.1em .6em
0&\mapright{}&{\cal O}_C&\mapright{}&\pi_*{\cal O}_D&
\mapright\delta&R^1f_*{\cal O}_V(-D)&\mapright{}&0\cr
&&&&&&\mapdown{\theta_i\otimes\gamma^{-1}}\cr
\rule 0pt 1.1em .6em
&&&&&&\hidewidth R^1f_*\bigl({\cal O}_V(-iM)\bigr)\otimes\gamma^{-1}\hidewidth\cr
&&&&&&\mapdown{}\cr
&&&&&&0
}