Why was the son-in-law called "the son-in-law in the east bed" in ancient times?

"The son-in-law in the east bed" is a famous idiom story in ancient China. The hero of the story is Wang Xizhi.

Wang Xizhi, a nephew of Wang Dao, a politician in the Eastern Jin Dynasty, often went in and out of the prime minister's mansion of his uncle Wang Dao and got together with his elders and brothers.

Chi Jian and Wang Dao, both ministers of the Eastern Jin Dynasty, have always had a good relationship.

Chi Jian has a daughter named Xi Zhuo, who is knowledgeable, talented and beautiful, and loves her like the apple of her eye. At marriageable age, Chi Jian wants to choose a good son-in-law for her daughter.

Chi Jian heard that there are many children in Prime Minister Wang Dao's family, all of whom are excellent in talent and appearance. Therefore, Chi Jian hopes to choose a husband among the children of Prime Minister Wang's family. Wang Dao, the prime minister, was very happy, of course, and promised to let Chi Jian go to his home and pick whatever he chose.

Chi Jian sent a housekeeper with a generous gift to the Prime Minister Wang's house.

The children of the Wangfu heard that Captain Xi sent someone to look for a son-in-law. They all dressed up carefully and came out to meet each other, hoping to choose themselves.

Looking at the selection of the housekeeper in Longfu, I feel that the young talents in Wangfu are all elites.

Finally, when the housekeeper of Longfu came to the study room of the East Yard, he found a young man lying on the bed against the wall, with his chest bare, sweating, drinking tea and thinking about things.

it turns out that this young man is Wang Xizhi. On the way to Xiangfu, Wang Xizhi saw the ancient tablet of Cai Yong, a famous calligrapher in the Eastern Han Dynasty, so he enjoyed it again and again, indulged in it and forgot all about the blind date. When he hurried to Xiangfu, he took off his coat because it was too hot ...

The housekeeper was very surprised to see him dressed like this, and he didn't answer when he asked him. It turned out that Wang Xizhi at this time was still thinking about Cai Yong's calligraphy!

the housekeeper of Xi mansion returned to the mansion and told Xi Taiwei what she had just seen and heard in the palace.

Chi Jian was very interested in the young man who the housekeeper said was "topless" and decided to choose him as his son-in-law.

when Chi Jian came to the palace, he saw that this man was both talented and handsome, open-minded and elegant. He paid a dowry on the spot and chose him as his husband. This is the origin of the "son-in-law".