UK TeX FAQ -- question label atvert

### `\``spacefactor` complaints

The errors

>     ! You can't use `\spacefactor' in vertical mode.
>     \@->\spacefactor
>                      \@m

or

>     ! You can't use `\spacefactor' in math mode.
>     \@->\spacefactor
>                      \@m

or simply

>     ! Improper \spacefactor.
>     ...

bite the LaTeX programmer who uses an internal command without taking “precautions”. An internal-style command such as `\``@foo` has been defined or used in a private macro, and it is interpreted as `\``@`, followed by the ‘text’ `foo`. (`\``@` is used, for real, to set up end-of-sentence space in some circumstances; it uses `\``spacefactor` to do that.)

The problem is discussed in detail in “`@` in macro names”, together with solutions.

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