UK TeX FAQ -- question label plninltx*

### Using Plain or primitive commands in LaTeX

It’s well-known that LaTeX commands tend to be more complex, and to run more slowly than, any Plain TeX (or primitive) command that they replace. There is therefore great temptation not to use LaTeX commands when macro programming. Nevertheless, the general rule is that you should use LaTeX commands, if there are seeming equivalents. The exception is when you are sure you know the differences between the two commands and you know that you need the Plain TeX version. So, for example, use `\``mbox` in place of `\``hbox` unless you know that the extras that LaTeX provides in `\``mbox` would cause trouble in your application. Similarly, use `\``newcommand` (or one of its relatives) unless you need one of the constructs that cannot be achieved without the use of `\``def` (or friends).

As a general rule, any LaTeX text command will start a new paragraph if necessary; this isn’t the case with Plain TeX commands, a fact which has a potential to confuse.

The commands `\``smallskip`, `\``medskip` and `\``bigskip` exist both in Plain TeX and LaTeX, but behave slightly differently: in Plain TeX they terminate the current paragraph, but in LaTeX they don’t. The command `\``line` is part of picture mode in LaTeX, whereas it’s defined as “`\``hbox` `to` `\``hsize`” in Plain TeX. (There’s no equivalent for users of the Plain TeX command in LaTeX: an equivalent appears as the internal command `\``@@line`).

Maths setting shows a case where the LaTeX version is essentially equivalent to the TeX primitive commands: the LaTeX `\``(``...` `\``)` does essentially no different to the TeX `$ ... $` (except for checking that you’re not attempting to open a maths environment when you’re already in one, or vice versa). However, `\``[``...` `\``]` isn’t the same as `$$ ... $$`: the TeX version, used in LaTeX, can miss the effect of the class option `fleqn`.

Font handling is, of course, wildly different in Plain TeX and LaTeX. Plain TeX’s font loading command (`\``font``\``foo=`<fontname>) and its LaTeX equivalent (`\``newfont`) should be avoided wherever possible. They are only safe in the most trivial contexts, and are potential sources of great confusion in many circumstances. Further discussion of this issue may be found in “What’s wrong with `\``newfont`?”.

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