Wrap avertising (also a mobile billboard) is the marketing practice of completely covering (wrapping) a vehicle in an advertisement or livery, thus turning it into a mobile billboard. This can be achieved by simply painting the vehicle surface, but it is becoming more common today to use large vinyl sheets as decals. These can be removed with relative ease, making it much less expensive to change from one advertisement to another. Vehicles with large, flat surfaces, such as buses and light-rail carriages, are fairly easy to work with, though smaller cars with curved surfaces can also be wrapped in this manner.