Commonly, billboard posters are printed on a high quality 120gsm blue backed paper that is suitable for paste application. This heavier than standard paper weight also offers easy handling and higher tear resistance. These posters are blue backed to reduce the show through of those covered. 
When an advertisement expires and there is no demand for its replacement, operators are responsible for covering it using the reverse of leftover posters. This procedure ensures that no advertisement remains on display beyond the agreed termination.
While Italy hit an economic recession the demand for billboard advertisements grew unpopular.
During a cloudy winter day all billboards in Milan were covered with the back side of the poster paper Blue Back. The blue shade of the inverted paper resembles the same blue of a sunny sky. That day, the sky was sunny and cloudy at the same time, in a city where the numerous billboards create the illusion of an artificial sky under the real one.