UK TeX FAQ -- question label whatmacros

### What are (TeX) macros

TeX is a macro processor: this is a computer-science-y term meaning “text expander” (more or less); TeX typesets text as it goes along, but expands each macro it finds. TeX’s macros may include instructions to TeX itself, on top of the simple text generation one might expect.

Macros are a good thing, since they allow the user to manipulate documents according to context. For example, the macro `\``TeX` is usually defined to produce “TEX” with the ‘E’ lowered, but in these FAQs the default definition of the macro is overridden, and it simply expands to the letters “TeX”. (You may not think this a good thing, but the author of the macros has his reasons – see TeX-related logos.)

Macro names are conventionally built from a `\` followed by a sequence of letters, which may be upper or lower case (as in `\``TeX`, mentioned above). They may also be `\<any single character>`, which allows all sorts of oddities (many built in to most TeX macro sets, all the way up from the apparently simple ‘`\``` ’ meaning “insert a space here”).

Macro programming can be a complicated business, but at their very simplest they need little introduction — you’ll hardly need to be told that:

>     \def\foo{bar}

replaces each instance of `\``foo` with the text “bar”. The command `\``def` is Plain TeX syntax for defining commands; LaTeX offers a macro `\``newcommand` that goes some way towards protecting users from themselves, but basically does the same thing:

>     \newcommand{\foo}{bar}

Macros may have “arguments”, which are used to substitute for marked bits of the macro expansion:

>     \def\foo#1{This is a #1 bar}
>     ...
>     \foo{2/4}

or, in LaTeX speak:

>     \newcommand{\foo}[1]{This is a #1 bar}
>     ...
>     \foo{3/4}

Macro writing can get very complicated, very quickly. If you are a beginner (La)TeX programmer, you are well advised to read something along the lines of the TeXbook; once you’re under way, TeX by Topic is possibly a more satisfactory choice. Rather a lot of the answers in these FAQs tell you about various issues of how to write macros.

This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=whatmacros