《文心雕龙·书记》
古代还有丹书铁券，用以确保信誓。汉代王褒的《僮约》，可说是约券的楷模了。
古有铁券，以坚信誓；王褒髯奴，则券之谐也。
Ancient times also had vermilion documents and iron coupons, used to confirm vows. Wang Bao of the Han dynasty's "Boy's contract" can be said to be a model contract.