Your own commands with \newcommand

Write \newcommand{\name}{replacement} in the preamble, with [n] for arguments #1 to #n; from then on \name is replaced by its definition, in text and in math.

A command without arguments

A command saves typing and keeps notation consistent. Define it once in the preamble and use it anywhere.

Shortcuts for sets and a product name

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\documentclass{article}
\usepackage{amssymb}
\newcommand{\R}{\mathbb{R}}
\newcommand{\product}{Scissor LaTeX}
\begin{document}
Every $x \in \R$ has a square $x^2 \ge 0$.
This page was typeset by \product.
\end{document}

Arguments and a default

[n] after the name says how many arguments the command takes; inside the definition they are #1, #2 and so on. A second bracket gives the first argument a default, which makes it optional.

\documentclass{article}
\newcommand{\vect}[1]{\mathbf{#1}}
\newcommand{\norm}[1]{\left\lVert #1 \right\rVert}
\newcommand{\greet}[2][Hello]{#1, #2!}
\begin{document}
The length of $\vect{v}$ is $\norm{\vect{v}}$.
\greet{Erika} \greet[Good morning]{John}
\end{document}

Changing an existing command

\renewcommand replaces a command that already exists, and \providecommand defines one only if it does not exist yet. \newcommand for a name that is taken is an error in a TeX installation; use \renewcommand then.

\documentclass{article}
\renewcommand{\emph}[1]{\textbf{#1}}
\providecommand{\R}{\mathbb{R}}
\begin{document}
This is \emph{emphasised} in bold now, and $\R$ is defined.
\end{document}

Operator names

Function names such as sin and log are set upright with the right spacing. \DeclareMathOperator (amsmath) makes your own; the starred form puts limits underneath in displays, like \max.

\documentclass{article}
\usepackage{amsmath}
\DeclareMathOperator{\Var}{Var}
\DeclareMathOperator*{\argmax}{arg\,max}
\begin{document}
\[
  \Var(X) = \mathbb{E}\left[(X - \mu)^2\right],
  \qquad
  \hat{\theta} = \argmax_{\theta} L(\theta)
\]
\end{document}

Your own environments

\newenvironment{name}[n]{before}{after} defines an environment: the "before" code runs at \begin{name} and the "after" code at \end{name}. Arguments can be used in the "before" part.

\documentclass{article}
\newenvironment{task}[1]
  {\par\noindent\textbf{Task #1.}\ }
  {\par}
\begin{document}
\begin{task}{44}
Measure the rainfall every morning.
\end{task}
\end{document}

What works

Definitions are expanded before typesetting, so a command can be used in text, in math, in captions and in section titles. To protect against loops, expansion stops at a safety limit, with a message in the Problems tab.

xparse commands

\NewDocumentCommand from xparse is not read. The definition is dropped, and where the command is used, its name is skipped with a message while its arguments stay in the text.

\documentclass{article}
\usepackage{xparse}
\NewDocumentCommand{\pair}{m m}{(#1, #2)}
\begin{document}
The pair \pair{a}{b} loses its parentheses and comma.
\end{document}

To fix it, write the same command with \newcommand{\pair}[2]{(#1, #2)}. For optional arguments, \newcommand with a default covers the common case.

What differs from a TeX installation

The xparse and mathtools manuals are on CTAN: xparse, mathtools. For operators and function names, see also Sums, integrals and function names.

Every example on this page is typeset by Scissor LaTeX when the site is built. Last updated 2026-11-07.