Berdelism is a form of marriage that is an alternative to financial transaction such as giving a dower, dowry or bridewealth. In the case of a maiden or bachelorette within one's household, she is given in exchange for a female relative in the receiving household. In the event of a bachelor within one's household, he is given in exchange for a male relative in the receiving household. In other words, you either exchange males, or exchange females. So in essense, berdelist ceremonies always involve four people. This type of marriage is useful for people who are not financially well-off and thereby can't afford to pay huge sums of money for weddings.
Berdelism has many different names, including parallel marriage, four-way marriage, berdel mariage, sister swapping, daughter swapping, brother swapping or son-swapping, niece swapping or nephew swapping.