The Prairie School was a primarily residential architectural movement that began in Chicago yet rapidly spread across the Midwest. Ultimately its influence was felt around the world—most especially in north-central Europe and Australia. Its origins date from the 1890s. Its vitality was largely sapped during the First World War, when homebuilders' attitudes turned conservative and thereafter shunned building concepts that expressed an idea rather than traditional architectural forms—ideas such as the relevance of a building to nature and the landscape, the visual expression of natural materials (rather than concealing them behind paint and wallpaper), or the idea of abandoning small, boxy rooms in favor of a more open, integrated interior space.