no effect of \baselinestretch

I'm probably doing something wrong, but i can't see any effect of the command \baselinestretch, which should modify the vspace between lines. I read somewhere, that i should add \selectfont after it, but i can't see any effect either:

\documentclass[12pt]{article}
\begin{document}

{\renewcommand{\baselinestretch}{2}\selectfont coucou coucou coucou coucou coucou coucou coucou coucou coucou coucou coucou coucou coucou coucou coucou coucou coucou coucou coucou coucou coucou coucou coucou coucou coucou coucou coucou coucou coucou coucou coucou coucou coucou coucou coucou coucou }

\end{document}

-
You're issuing \par when the setting of \baselinestretch has been forgotten because the group has ended. Put \par before the closing brace. –  egreg Mar 27 '12 at 7:59
May be tex.stackexchange.com/questions/48741/… is of some help? –  Harish Kumar Mar 27 '12 at 8:14
ok, thanks a lot. –  nicolas roy Mar 27 '12 at 8:15

This problem is similar to when one writes

{\Large some long text
that will span two or
more lines}


and the line spacing comes out wrong.

TeX uses one baseline skip for all lines in a paragraph, using the value in force when it scans the end of paragraph. In the example and also in your code the \par (or equivalent blank line) is found when the group where the change has been issued has already been closed and its effect canceled. The following code

\documentclass[12pt]{article}
\begin{document}


won't have the problem. Notice the \linespread{2} command, which is handier than \renewcommand{\baselinestretch}{2} and has the same effect.