Code for labelling a parallel pair:

\overset{f}{\underset{g}{\rightrightarrows}}

$$\overset{f}{\underset{g}{\rightrightarrows}}$$

Code for a square 2-cell:

\require{AMScd} \begin{CD} A @>>> B\\ @VVV \Rightarrow @VVV\\ C @>>> D \end{CD}

$$\require{AMScd} \begin{CD} A @>>> B\\ @VVV \Rightarrow @VVV\\ C @>>> D \end{CD}$$

Code for a naturality square:

\require{AMScd} \begin{CD}
FA @>{\eta_A}>> GA\\ @V{Ff}VV @VV{Gf}V\\
FB @>>{\eta_B}> GB
\end{CD}

$$\require{AMScd} \begin{CD} FA @>{\eta_A}>> GA\\ @V{Ff}VV @VV{Gf}V\\ FB @>>{\eta_B}> GB \end{CD}$$

Code for a small chain map:

\newcommand{\ra}[1]{\kern-1.5ex\xrightarrow{\ \ #1\ \ }\phantom{}\kern-1.5ex}
\newcommand{\ras}[1]{\kern-1.5ex\xrightarrow{\ \ \smash{#1}\ \ }\phantom{}\kern-1.5ex}
\newcommand{\da}[1]{\bigg\downarrow\raise.5ex\rlap{\scriptstyle#1}}
\begin{array}{c}
0 & \ra{f_1} & A & \ra{f_2} & B & \ra{f_3} & C & \ra{f_4} & D & \ra{f_5} & 0 \\
\da{g_1} & & \da{g_2} & & \da{g_3} & & \da{g_4} & & \da{g_5} & & \da{g_6} \\
0 & \ras{h_1} & 0 & \ras{h_2} & E & \ras{h_3} & F & \ras{h_4} & 0 & \ras{h_5} & 0 \\
\end{array}

$$\newcommand{\ra}[1]{\kern-1.5ex\xrightarrow{\ \ #1\ \ }\phantom{}\kern-1.5ex} \newcommand{\ras}[1]{\kern-1.5ex\xrightarrow{\ \ \smash{#1}\ \ }\phantom{}\kern-1.5ex} \newcommand{\da}[1]{\bigg\downarrow\raise.5ex\rlap{\scriptstyle#1}} \begin{array}{c} 0 & \ra{f_1} & A & \ra{f_2} & B & \ra{f_3} & C & \ra{f_4} & D & \ra{f_5} & 0 \\ \da{g_1} & & \da{g_2} & & \da{g_3} & & \da{g_4} & & \da{g_5} & & \da{g_6} \\ 0 & \ras{h_1} & 0 & \ras{h_2} & E & \ras{h_3} & F & \ras{h_4} & 0 & \ras{h_5} & 0 \\ \end{array}$$

Code for a table:

\begin{array}{|c|c|c|c|}
\hline
t & a & b & l & e \\
\hline
a & b & c & d & e \\
a & b & c & d & e \\
a & b & c & d & e \\
a & b & c & d & e \\
\hline
\end{array}

$$\begin{array}{|c|c|c|c|} \hline t & a & b & l & e \\ \hline a & b & c & d & e \\ a & b & c & d & e \\ a & b & c & d & e \\ a & b & c & d & e \\ \hline \end{array}$$