《礼记·王制》
命服的规定：天子的三公本已八命，再加一命成九命，就可以穿衰衣了。
制：三公，一命卷；若有加，则赐也。
The regulations regarding rank in life: the emperor's three dukes were already eight-ranked and then were given a ninth rank and thus could wear embroidered clothing.