CSS trick - reveal an area when printed

I had to complete some work for a client where they wanted an additional area where the employee could sign off the printed version of a form. They did not want the viewable version of the form to display the signature area until the document was printed. This is the code that i used to present a block of code however only when the form was printed. It worked really well as in the web browser you could just see the form where as when it was printed the additional signature area displayed at the bottom of the page. This code is probably quiet useful to some so I though that I would post it.