《旧唐书·列传·卷十五》
遥想夏商周三代，习俗无常，于是设立公侯伯子男五等爵号，顺应时势作为条例。
缅惟三代，习俗靡常，爰制五等，随时作教。
When looking at the three dynasties of Xia, Shang and Zhou, the customs and habits were inconsistent, so the titles of duke, marquis, earl, viscount and baron were established to follow the times and set rules.