After Emperor Gaozu of Han, Liu Bang, established the Han Dynasty, he enfeoffed many princes, and his nephew Liu Pi was granted the title of King of Wu.
During the Han Dynasty, the feudal princes wielded immense power within their own kingdoms—they could levy taxes, mint coins, and appoint officials below the rank of prime minister. Over time, their influence swelled, and they grew increasingly defiant toward the central court. Some ambitious princes even plotted rebellions, posing a grave threat to the empire.
After Liu Bang's death, Emperor Wen and Emperor Jing of Han gradually reduced the fiefdoms of vassal kings. Liu Bi and other princes, deeply resentful, secretly plotted rebellion against the central court.
At that time, several key advisors under Liu Bi, including Zou Yang, Yan Ji, and Mei Cheng, opposed the rebellion and submitted memorials urging the King of Wu to reconsider. Mei Cheng analyzed in his memorial:
"If a thousand-jun weight were suspended from a single thread over a bottomless abyss, even the dullest fool would recognize the extreme danger. If a startled horse were then frightened again by beating drums, with a sheer chasm ahead, the horse would surely plunge into the abyss. If the thread is about to snap and more weight is added, or the horse is already spooked and drums are beaten again, the thread will break beyond repair and the horse will fall beyond rescue. The peril of such a situation is like two points so close together that not even a single hair can fit between them. I beg Your Majesty to ponder this deeply."
However, the Prince of Wu, Liu Pi, ignored the wise counsel, so Mei Cheng and his associates left him to pledge allegiance to Liu Wu, the Prince of Liang and younger brother of Emperor Jing.
In 154 BCE, Liu Pi, the King of Wu, rallied the kings of Chu, Zhao, and four other states to launch a rebellion, historically known as the "Revolt of the Seven States." Emperor Jing of Han sent Grand Commandant Zhou Yafu to suppress the rebellion, which was quelled in just three months. Some rebel kings committed suicide, others were killed, and Liu Pi himself was slain by the Yue people after fleeing to Dongyue.
Later, the idiom "Not a Hair's Breadth Between" came to describe a situation of extreme danger.
Source: *Memorial to the King of Wu* by Mei Cheng (Han Dynasty)
Meaning of the Idiom: Later, the Chinese idiom "间不容发" came to describe a situation of extreme danger.