LaTeX error · Commands, braces and arguments

LaTeX Error: Command \R already defined.

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.

! LaTeX Error: Command \R already defined.
               Or name \end... illegal, see p.192 of the manual.

See the LaTeX manual or LaTeX Companion for explanation.
Type  H <return>  for immediate help.
 ...                                              
                                                  
l.4 \newcommand{\R}{\mathbb{R}}

Why it happens

• A name that a package already uses, for example \R, \C or \G from some packages, or \vec, \comment and \proof.
• The same \newcommand twice, often because two included files or a template and your preamble both define it.
• A name starting with \end, which \newcommand rejects (Or name \end... illegal).

How to fix it

Rename your macro to something unused (for example \Real or \reals), or, when you deliberately want your own version, use \renewcommand. Check what the old command did before replacing it: overwriting something like \vec or \c can break other parts of the document.

Broken: reproduces the error

\documentclass{article}
\usepackage{amssymb}
\newcommand{\R}{\mathbb{R}}
\newcommand{\R}{\mathbb{R}}

\begin{document}

Let $x \in \R$.

\end{document}

Fixed: compiles cleanly

\documentclass{article}
\usepackage{amssymb}
\providecommand{\R}{}
\renewcommand{\R}{\mathbb{R}}

\begin{document}

Let $x \in \R$.

\end{document}

Related errors

• Undefined control sequence – A command LaTeX does not know: misspelled, or its package is not loaded.
• Illegal parameter number in definition – A macro body uses #2 (or #) for an argument that the definition does not declare.
• Option clash for package – A package is loaded twice with different options, often once by another package.
• All LaTeX errors and warnings – The full list, grouped by cause

Frequently Asked Questions

What is the difference between \newcommand and \renewcommand?

\newcommand only defines commands that do not exist and errors otherwise. \renewcommand only redefines commands that do exist and errors if the name is unknown. \providecommand defines it only when it is missing.

How do I check whether a command is already defined?

Put \show\R (with your command name) in the preamble and compile. The log shows undefined if the name is free, or its current definition otherwise.

Why does a template give Command already defined?

The template and a package you added define the same name, or the class already provides it. Remove your duplicate definition or switch it to \renewcommand.