Roman theorems

If you want to take advantage of the powerful \newtheorem command without the constraint that the contents of the theorem is in a sloped font (for example, to use it to create remarks, examples, proofs, ...) then you can use the AMSLaTeX amsthm package (which now supersedes the theorem package previously recommended in these answers). Alternatively, the following sets up an environment remark whose content is in roman.

  \newtheorem{preremark}{Remark}
  \newenvironment{remark}%
    {\begin{preremark}\upshape}{\end{preremark}}

amsthm.sty

Distributed as part of macros/latex/required/amslatex (zip, browse)

ntheorem.sty

macros/latex/contrib/ntheorem (zip, browse)

theorem.sty

Distributed as part of macros/latex/required/tools (zip, browse)

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