The predominant shape for mirrors until the Tang dynasty was round. During the Tang more shapes appear, including lobed and, as in this case, square. This unusual example features a crane, a phoenix and two flying deities, surrounded by cloud scrolls. Cloud designs relate to the belief that the soul could travel on a cloud to join the world of immortal beings.
Square flying deity and crane mirror Tang dynasty (AD 618-906) © Compton Verney