Collage generally refers to two-dimensional works created from an assemblage of different materials, although the distinction between collage and assemblage (which refers to three-dimensional works) can be quite fluid. Collage was an assault on painting and its proposal of imaginary space as well as the proper materials of art. The development of collage in art ran parallel to experiments in poetry and literature in the early 20th century, where aesthetic principles of fragmentation, reassembly, and multiple meanings reflected the rapid societal and technological changes ushered in by modernism and urbanization.