《史记·三十世家·楚世家》
十一年，齐王秦王各自称帝，一个月后，又把帝改为王。
十一年，齐秦各自称为帝；月余，复归帝为王。
In the eleventh year, the Qi king and the Qin king called themselves emperors respectively, and a month later, they changed it back to the king.