This was based on the description in the king system part of Raiki (Book of Rites), one of Gokyo (Five Classics), that "The salary and title in the king's system is commonly classified into five ranks including Ko (公), Ko (候), Haku, Shi, Dan."
これは五経の一つである礼記の王制篇に『王者之制禄爵　公候伯子男　凡五等』とあるのにならったものである。