During the early Spring and Autumn period, the state of Jin annexed several smaller states and grew into a major power. To strengthen ties with the equally powerful state of Qin, Duke Xian of Jin married his eldest daughter to Duke Mu of Qin—she became known in history as Lady Mu of Qin.
Duke Xian of Jin, in his old age, doted on his concubine Li Ji. Heeding her slanderous words, he forced Crown Prince Shensheng to his death. Li Ji then plotted against Princes Yiwu and Chong'er, aiming to secure the throne for her own son, Xiqi. Fearing for their lives, Yiwu and Chong'er had no choice but to flee the state of Jin.
In 651 BCE, Duke Xian of Jin died, and his favored concubine Li Ji saw her son Xi Qi installed as ruler—only for him to be killed shortly after by two ministers loyal to another prince, Yiwu. They then sent envoys to summon Yiwu from exile in Liang to return and take the throne.
Yiwu feared he could not control the situation upon returning to Jin, so he asked Duke Mu of Qin to send troops to escort and support him, promising to cede five cities to Qin in return. However, after taking the throne (historically known as Duke Hui of Jin), he broke his promise, greatly angering Duke Mu. Four years later, when a famine struck Jin, they sought aid from Qin. Duke Mu, setting aside past grievances, sent large amounts of grain to help Jin survive the crisis. Yet the following year, when Qin faced its own famine, Duke Hui refused to provide any grain. A year later, Duke Mu led an army to attack Jin and captured Duke Hui alive. With the help of Lady Mu of Qin, Duke Mu not only pardoned Duke Hui but also forged a covenant with him.
After surviving this ordeal, Duke Hui of Jin strengthened ties with Qin by sending his crown prince, Yu, as a hostage to Qin, where Duke Mu of Qin married his daughter, Huai Ying, to Yu. Unexpectedly, Yu later fled back to Jin behind Duke Mu's back, and when Duke Hui died the following year, Yu ascended the throne as Duke Huai of Jin. Duke Huai was harsh by nature, arbitrarily killing senior ministers, which sparked intense resentment among the court officials.
After nineteen years of exile wandering through various states, Prince Chong'er of Jin finally arrived in the state of Qin. Duke Mu of Qin, impressed by his talent and integrity, married five royal women to him, including Huai Ying. One day, Huai Ying held a basin for Chong'er to wash his hands. After finishing, he dismissively waved her away. Huai Ying angrily demanded, "Qin and Jin are equal states—why do you insult me?"
Chong'er realized his mistake and immediately apologized to her. Later, Duke Mu of Qin sent troops to escort Chong'er back to Jin, where he had Huai Gong assassinated, and the ministers all supported him as ruler. Afterwards, he arranged for the crown prince to marry a Qin princess, thus forging marital ties between father and son with Qin, forming the "Qin-Jin Alliance."
Later, the idiom "the bond of Qin and Jin" came to be used as a metaphor for marriage between two families.
Source: *Zuo Zhuan*, "Duke Xi's Twenty-Third Year"
Meaning of the Idiom: Later, the Chinese idiom "秦晋之好" came to describe a metaphor for marriage between two families.