Labels and cross-references

Never write "see Section 3" by hand

• If you write a number by hand and later add a section, every number is wrong.
• LaTeX solves this with labels. Tag something:

\section{Methods}\label{sec:methods}

• The name sec:methods is just a nickname you choose.

Refer to it anywhere

• Then point at it with \ref — LaTeX inserts the current number:

As shown in Section~\ref{sec:methods}, ...

• The ~ is a space that never breaks across lines, so "Section" and its number stay together.

It needs two passes

• On the first compile LaTeX learns the numbers; on the second it fills them in.
• Our Run does this for you automatically. (The same trick powers a table of contents.)

Common mistakes

• A typo in the key. \ref{sec:b} when you wrote \label{sec:a} prints ??, and the log says LaTeX Warning: Reference 'sec:b' on page 1 undefined. It is a warning, so the PDF still builds: look for ??.
• \label before the thing it names. Put it after \section{...} (or after \caption{...} in a figure), or it picks up the wrong number.
• One compile only. The first run writes the numbers down; the second uses them. ?? after one run is normal.