Commands

Meaning of \quad
Put comment lines
Put matrices
Label \(Ax = b\) under a matrix
Put horizontal spaces
Mathematical Symbols

Meaning of `\quad` discussion

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

https://tex.stackexchange.com/questions/119068/meaning-of-quad

Put comment lines howto

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

https://tex.stackexchange.com/questions/17816/commenting-out-large-sections

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}

\[\begin{equation} \left[\begin{array}{cc|c} 1 & 2 & 3\\ 4 & 5 & 6 \end{array}\right] \end{equation} \]

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}

\[\begin{align} \mathop{\begin{pmatrix} a & b \\ c & d \end{pmatrix}}_{\textstyle A} \times \mathop{\begin{pmatrix} x1 \\ x2 \end{pmatrix}}_{\textstyle x\vphantom{A}} = \mathop{\begin{pmatrix} b1 \\ b2 \end{pmatrix}}_{\textstyle b\vphantom{A}} \end{align} \]