Seal script (zhuanshu, 篆書) exists in two major forms. The earlier form, known as large-seal script (dazhuan, 大篆), derived from symbols cast on bronze ritual vessels from the Shang and Zhou dynasties of the eleventh to third century BC. As its linear composition became more regular, seal-script inscriptions were used mostly for commemorative records. Its later and more unified form, called small-seal script (xiaozhuan, 小篆), was specifically devised as a standardized system of writing under the first emperor of the Qin dynasty, who reigned from 221 to 209 BC. Often used for official inscriptions on stone monuments, small-seal script is characterized by a symmetrical structure formed with thin, even lines executed with balanced movements.