LaTeX error · Commands, braces and arguments

Undefined control sequence.

The error in the log

This is how the message appears in the .log file (and in the compile output). The line starting with l. is the line number where TeX noticed the problem; the real mistake is there or just before it.

! Undefined control sequence.
l.5 The set \mathbb
                   {R} of real numbers.

Why it happens

• A typo in the command name: \textbol, \beign, \fraq, or a wrong capital letter (\Alpha does not exist in standard LaTeX).
• The command belongs to a package that is not loaded: \mathbb and \mathfrak (amssymb), \includegraphics (graphicx), \url (url or hyperref), \SI (siunitx), \toprule (booktabs).
• A backslash in text that was meant literally, such as a Windows path C:\Users\data or a stray \ before a word.
• A command defined only in one class or template (for example \email or \affiliation) used with a different document class.

How to fix it

Read the line after the error: the log breaks it exactly after the unknown command, so the last token on the l.<n> line is the culprit. Here it is \mathbb, which is defined by the amssymb package, not by LaTeX itself.

If the spelling is right, search for the package that provides the command and load it in the preamble. If the backslash was meant to be printed, write \textbackslash instead.

Broken: reproduces the error

\documentclass{article}

\begin{document}

The set \mathbb{R} of real numbers.

\end{document}

Fixed: compiles cleanly

\documentclass{article}
\usepackage{amssymb}

\begin{document}

The set $\mathbb{R}$ of real numbers.

\end{document}

For a longer walkthrough with more examples, read Undefined control sequence in LaTeX: causes and fixes on the blog.

Related errors

• Missing $ inserted – A math-only symbol (_, ^, \alpha) was used in text, or math mode was left early.
• Environment undefined – A \begin{...} name LaTeX does not know: usually a missing package or a typo.
• File not found – A package or class named in \usepackage or \documentclass is misspelled or not installed.
• All LaTeX errors and warnings – The full list, grouped by cause

Frequently Asked Questions

How do I find which command is undefined?

Look at the line in the log that starts with l. and a number. TeX splits it at the point where it stopped, so the last command on the first half is the undefined one. The number is the line in your source file.

Why do I get Undefined control sequence for \mathbb or \includegraphics?

These commands are not part of core LaTeX. Add \usepackage{amssymb} for \mathbb and \usepackage{graphicx} for \includegraphics to the preamble, before \begin{document}.

Can I ignore Undefined control sequence and still get a PDF?

In nonstop mode LaTeX skips the unknown command and keeps going, so a PDF may appear, but whatever the command should have produced is missing. Fix the error rather than relying on that output.

Why does the error point to the preamble?

A command used in the preamble, such as \setlength{\parskip} with a misspelled length or a \newcommand body that uses an unknown macro, is reported there. If the error appears when a macro you defined is used, the unknown command is inside your definition.