UK TeX FAQ -- question 252

### What's going on in my `\include` commands?

The original LaTeX provided the `\include` command to address the problem of long documents: with the relatively slow computers of the time, the companion `\includeonly` facility was a boon. With the vast increase in computer speed, `\includeonly` is less valuable (though it still has its place in some very large projects). Nevertheless, the facility is retained in current LaTeX, and causes some confusion to those who misunderstand it.

In order for `\includeonly` to work, `\include` makes a separate `.aux` file for each included file, and makes a 'checkpoint' of important parameters (such as page, figure, table and footnote numbers); as a direct result, it must clear the current page both before and after the `\include` command. What's more, this mechanism doesn't work if a `\include` command appears in a file that was `\include`d itself: LaTeX diagnoses this as an error.

So, we can now answer the two commonest questions about `\include`:

- Why does LaTeX throw a page before and after `\include` commands?

  Answer: because it has to. If you don't like it, replace the `\include` command with `\input` - you won't be able to use `\includeonly` any more, but you probably don't need it anyway, so don't worry.

- Why can't I nest `\include`d files? - I always used to be able to under LaTeX 2.09.

  Answer: in fact, you couldn't, even under LaTeX 2.09, but the failure wasn't diagnosed. However, since you were happy with the behaviour under LaTeX 2.09, replace the `\include` commands with `\input` commands (with `\clearpage` as appropriate).