Sister exchange is a type of marriage agreement where two sets of siblings marry each other. In order to get married, a man needs to persuade his sister to marry the bride's brother. It is practised as a primary method of organising marriages in 3% of the world's societies: in Australia, Melanesia, Amazonia and Sub-Saharan Africa; and can replace other methods in 1.4% of the societies. Despite earlier claims of its simplicity, sister exchange is a complex arrangement that involves many family members and not simply the four people who are getting married.