《礼记·杂记上》
国君去世的当年，太子只称 子 ，但在交往中受到的礼遇却和国君一样。
君薨，大子号称子，待犹君也。
In the year when the sovereign passes away, the Crown Prince is only called "Crown Prince", but he receives the same courtesies as the sovereign in social interactions.