There was a custom in my birthplace which came into being for the difficulty of marriage. When a family had one son and one daughter at the age of marriage, its head would never marry his daughter off easily before he found a wife for his son. As soon as he realized it impossible for his son to look for a partner in the usual way, he would adopt the last method to change the situation. That was to exchange daughters with another similar family facing the same difficulty through a matchmaker. In this way, the sons of the two families got married by taking each other's daughters as wives. There were also such exchanges among three families.