Art historians use several ways to put together systems of classification for the world's arts. They make groups out of the continuous flow of art through time and space. These groups are based on the idea that their works share a single important quality or a set of essential attributes. Important qualities show how an artist approaches their work. They can be formal, stylistic, iconographic, thematic, or something else. The definition of a grouping shows how people think about the connections between artworks and between artworks and the world around them. Western art is usually divided into periods based on art movements, primarily based on cultural and aesthetic criteria. Eastern art, on the other hand, is divided into periods based on political dynasties.