After Qin conquered the last of the rival states in 221 BC, the First Emperor imposed one system on the whole realm. He set up commanderies and counties everywhere and unified weights and measures. His measures are remembered in the phrase che tong gui, shu tong wen: carts on the same gauge, writing in the same script.
The six conquered states had each written in their own way. On the advice of his minister Li Si, the emperor abolished their scripts and made the small seal script the standard for the whole country.