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— title: `\spacefactor` complaints category: errors permalink: /FAQ-atvert —
The errors ```latex ! You can't use `\spacefactor' in vertical mode. \@→\spacefactor
``` or ```latex ! You can't use `\spacefactor' in math mode. \@→\spacefactor
``` or simply ```latex ! 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](FAQ-atsigns)”, together with solutions.