Lei Chi, an ancient place name, is now located in Wangjiang County, Anhui Province. In the Eastern Jin Dynasty, a thunder pool was set up, which was named after the pool was near the thunder water (now Leigang). This is the origin of "one step without crossing the line" in Yu Liang's Letter to Wen Qiao.
In addition, according to the Book of Jin, Yu Liang was the secretariat of Jiang, Jing and Yu, guarding Wuchang (now Hubei). In the spring of the sixth year of Xiankang (340), after Wang Dun's death, Yu Liang's influence in Tao Kan was expanding day by day, and he wooed the Soviet army in Liyang (now Anhui County) town, making it the top general and magistrate of Liyang, and was allowed by the court to move the Soviet army to the town (now Dangtu, Anhui Province) to guard Jiankang (now Nanjing). This not only weakened the power of the Langya Wangs, but also put Yu Liang under the threat of Tao Kan's power. In order to avoid worries, Yu Liang sent two groups of messengers to contact the Soviet Union in the name of conveying hospitality and plotting rebellion. While ostensibly perfunctory, the Soviet army secretly assembled its men to prepare for rebellion. In May of the same year, Su Jun sent his brother Su Yi and his Ministry to lead the army into Jiankang. On the one hand, Yu Liang sent generals Mao Bao and Zhao Yin to fight the rebels in Cihu; On the one hand, he sent soldiers to Jiankang to guard, and at the same time, he issued an urgent order to Wenqiao to lead the army to fight together. Wen Qiao came to Jiankang from Jingzhou (now Jiangling, Hubei) after learning about it. When passing by Niuzhu, Yu Liang wrote a letter, warning Wen Qiao not to cross the line, in case Tao Kan stepped in. In fact, Yu Liang misunderstood Tao Kan. At that time, Tao Kan was preparing to send Gao Dan to join the army to go to Jiankang to participate in the war, but Gao Dan knew that Tao Kan and Yu Liang had contradictions, and when Mao Bao, who was closely related to Yu Liang, was defeated, he advised Tao Kan to send troops to Jiankang. After reaching an agreement on internal contradictions, Tao Kan led the army from Wuchang to the east. When passing through the thunder pool, Wenqiao sent a soldier to meet him on his birthday and conveyed Yu Liang's warning "Don't cross the thunder pool one step". After Tao Kan angered Gao's rudeness, he ordered people to swear by pointing to a penny by the river: "Only after breaking the thief can this be done." . And lead the east.
Therefore, the allusion of "dare not cross the line" mainly comes from an order of Yu Liang.