Guide to LaTeX (4th Edition) (Tools and Techniques

C.2. Some sample errors 411
The last can easily occur when a L AT E X file is to be read in with \input or
\include, but is mistakenly entered directly with the L AT E X program call.
If a file with the text
This file has no preamble.
is put directly into the L AT E X processing, the following message appears
on the monitor:
! LaTeX Error: Missing \begin{document}.
See the LaTeX manual or LaTeX Companion for explanation.
Type H for immediate help.
...
l.1 T
his file has no preamble.
?
with the accompanying help message after typing H 〈return〉:
You’re in trouble here. Try typing to proceed.
If that doesn’t work, type X to quit.
?
One sees from the error message that L AT E X has discovered a mistake
on reading the very first letter of the text. The help message is also not
very encouraging. There is no point trying to continue the processing
with 〈return〉. Rather one should stop it with X or E right away, since
nothing useful can be achieved here.
Even when the above example file contains the environment
\begin{document}
This file has no preamble.
\end{document}
it is not possible to carry out a proper processing. The T E X error message
now printed is
! LaTeX Error: The font size command \normalsize is not defined:
there is probably something wrong with the class file.
See the LaTeX manual or LaTeX Companion for explanation.
Type H for immediate help.
...
l.1 \begin{document}
?