# Meaning of \quaddiscussion

The word quad is a traditional term in typography. It comes from Italian quadratone (big square). In old fashioned metal typography it meant a square piece of metal lower than type height that could be inserted between types for spacing them.

• It's commonly as wide and high as an uppercase M,
• \qquad just means two quads

# Put comment lines howto

• Use \iffalse\fi to make LaTex not compile everything between it.

# Put matrices howto

\begin{matrix}
a & b \\
c & d
\end{matrix}
$\begin{matrix} a & b \\ c & d \end{matrix}$
\begin{pmatrix}
a & b \\
c & d
\end{pmatrix}
$\begin{pmatrix} a & b \\ c & d \end{pmatrix}$
\begin{bmatrix}
a & b \\
c & d
\end{bmatrix}
$\begin{bmatrix} a & b \\ c & d \end{bmatrix}$
\begin{vmatrix}
a & b \\
c & d
\end{vmatrix}
$\begin{vmatrix} a & b \\ c & d \end{vmatrix}$
\begin{Vmatrix}
a & b \\
c & d
\end{Vmatrix}
$\begin{Vmatrix} a & b \\ c & d \end{Vmatrix}$
\begin{equation}
\left[\begin{array}{cc|c}
1 & 2 & 3\\
4 & 5 & 6
\end{array}\right]
\end{equation}
$$$\left[\begin{array}{cc|c} 1 & 2 & 3\\ 4 & 5 & 6 \end{array}\right]$$$

# Label $$Ax = b$$ under a matrix howto

• Use \mathop
• \textstyle for a larger size and \vphantom for adjusting the baselines.
\begin{align}
\mathop{\begin{pmatrix}
a & b \\
c & d
\end{pmatrix}}_{\textstyle A}
\times
\mathop{\begin{pmatrix}
x1 \\
x2
\end{pmatrix}}_{\textstyle x\vphantom{A}}
=
\mathop{\begin{pmatrix}
b1  \\
b2
\end{pmatrix}}_{\textstyle b\vphantom{A}}
\end{align}
\begin{align} \mathop{\begin{pmatrix} a & b \\ c & d \end{pmatrix}}_{\textstyle A} \times \mathop{\begin{pmatrix} x1 \\ x2 \end{pmatrix}}_{\textstyle x\vphantom{A}} = \mathop{\begin{pmatrix} b1 \\ b2 \end{pmatrix}}_{\textstyle b\vphantom{A}} \end{align}

# Put horizontal spaces howto

Commands listed in ascending order
• \, (\thinmuskip)
• \> (\medmuskip)
• \; (\thickmuskip)
Some others
• \enspace inserts a space of .5em in text or math mode;
• \quad inserts a space of 1em in text or math mode;
• \qquad inserts a space of 2em in text or math mode;
• \kern <len>
• \hskip <len>
• \hspace{<len>}
• \hphantom{<stuff>}
• \hfill
• etc.