Numbers and pointers for equations

Numbering an equation is a promise that you will refer to it later. LaTeX handles both halves of that promise: the equation environment assigns the number, and the label system lets you point at it from anywhere in the document without ever typing the number yourself.

\begin{equation}
 \label{eq:bayes}
 p(\theta \mid y) \propto p(y \mid \theta)\, p(\theta)
\end{equation}
As shown in \eqref{eq:bayes}, ...

Labels and references

\begin{equation} numbers the displayed line. \label{eq:bayes} attaches the name eq:bayes to that number; the name is yours to choose, and an eq: prefix keeps equation labels distinct from figures and tables. To cite it, \ref{eq:bayes} prints the bare number, while \eqref{eq:bayes}, from amsmath, wraps it in parentheses to match how the number appears beside the equation. Prefer \eqref for equations so the two styles agree.

Because numbers are recorded on one pass and read on the next, a fresh compile shows ?? until the following run; question marks instead of numbers covers that cycle.

If a displayed equation is never referenced, do not number it. Use \[ ... \] or the starred equation* instead, so numbers mark only the lines a reader might need to find again.

Numbering some lines in align

Multi-line derivations in align number every line by default, which is rarely what you want. Two commands adjust that per line:

\begin{align}
 a &= b \label{eq:a} \\
 c &= d \notag \\
 e &= f \label{eq:e}
\end{align}

\notag suppresses the number on its line, so here the intermediate step stays unnumbered while the first and last lines keep their numbers and labels. Put each \label on the line it names, before the \\, so it binds to the right number. The all-or-nothing shortcut is align*, which numbers nothing; the full family of multi-line environments is covered in multi-line equations that line up.

The common mistake is writing the literal number, as in “see Equation 3”, which breaks the moment you insert an equation above it. Always go through a label and let LaTeX renumber on every compile. You can watch the numbering update live as you add and remove \notag in the playground.