In material design, the physical properties of paper are translated to the screen. The background of an application resembles the flat, opaque texture of a sheet of paper.
An application’s behavior mimics the paper’s ability to be re-sized, shuffled, and bound together in multiple sheets. Elements that live outside of applications, such as status or system bars, receive a different treatment. They are separate from the app content beneath them, and do not carry the physical properties of paper.
For detailed information about material, see Material Properties.
Two sheets of paper that share an entire common edge are called seams. While joined by a seam, they move together.
Two overlapping sheets of paper, with different z-positions (or depths), form a step. They move independently of each other.