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— title: `\spacefactor` complaints category: errors permalink: /FAQ-atvert date: 2014-06-10 —
# `\spacefactor` complaints
The errors ```latex ! You can't use `\spacefactor' in vertical mode. \@→\spacefactor
``` or ```latex ! You can't use `\spacefactor' in math mode. \@→\spacefactor
! Improper \spacefactor.
bite the LaTeX programmer who uses an internal command without
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](FAQ-atsigns),
together with solutions.