Rules / Build

Undefined control sequence — Unknown command

The document uses a command that no loaded package defines: a typo, a missing package, or a command from another citation package.

Fix

Fix the spelling or load the package that defines the command.

Example

Reproduced on TeX Live 2026 by CI · pdflatex · corpus/undefined-control-sequence

Typo in a command name

Fails: main.tex
\documentclass{article}
\begin{document}
\section{Results}
Our method is \textbff{three times faster} than the baseline.
\end{document}
Fixed
\documentclass{article}
\begin{document}
\section{Results}
Our method is \textbf{three times faster} than the baseline.
\end{document}
What TeX prints in main.log
! Undefined control sequence.
l.4 Our method is \textbff
                          {three times faster} than the baseline.
The control sequence at the end of the top line
of your error message was never \def'ed. If you have
misspelled it (e.g., `\hobx'), type `I' and the correct
spelling (e.g., `I\hbox'). Otherwise just continue,
and I'll forget about whatever was undefined.