Decorative paper that is applied in strips to the walls of a room. In Europe, wallpaper became popular in the 17th century. In the 18th century, machines were developed for printing wallpaper, and during the 19th century it became widely available in varieties that contained as many as 20 colors. While wallpaper was popular during the first third of the 20th century, Modernism’s preference for white walls led to its decline.