ad drop

A backdrop with advertisements for local businesses painted on it--providing additional revenue for the theatre owner. Often a roll drop, and also called an olio curtain, would be hung 6-10' US of the plaster line or main curtain line and comedians, jugglers, or other low maintenance acts could perform while scenery was being changed upstage.

A "modern" version:
paramountadscreen.jpg
http://www.theparamountcenter.com/about.html

See the wiki entry olio for a picture of a "classic" one, as well as more explanation.