% Fichier ex12.2.3-1.tex - Ch. Rolland/O Reilly France
\documentclass{article}
\usepackage[latin1]{inputenc}
\begin{document}
\fbox{Encadré}
{\setlength{\fboxrule}{2mm}
\fbox{Encadré}}
{\setlength{\fboxsep}{5mm}
\fbox{Encadré}}
\end{document}
