FormaTeX
FatalCommands & Packages

Undefined Control Sequence

! Undefined control sequence.

What this error means

LaTeX encountered a command (starting with \) that it doesn't recognise. This is one of the most common errors — it usually means a typo in a command name or a missing \usepackage.

Common causes

  • Typo in the command name (e.g. \textboldd instead of \textbf)
  • The package that defines the command is not loaded in the preamble
  • Using a command from a newer LaTeX version not available on the current installation
  • Forgot to define the command with \newcommand before using it

How to fix it

Check the command spelling carefully. If the command comes from a package, add \usepackage{packagename} to your preamble. Use the LaTeX companion or the package documentation to confirm the correct command name.

Code examples

Causes the error
\documentclass{article}
\begin{document}

\textboldd{This is bold text}

\end{document}
Fixed
\documentclass{article}
\begin{document}

\textbf{This is bold text}

\end{document}

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