After Qin Shi Huang unified the six states, he made small seal script the standard script of the whole country.
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history
小篆 was standardized around 221 BCE by the chancellor 李斯, who trimmed the older 大篆 into one national form. The Han dictionary 《说文解字》 is arranged by it, and 隶书 (clerical script) soon replaced it for everyday writing.