Prior to this, mothers were directly involved as partners in this arrangement however, after incest became more of a taboo, there was a shift to women who were not blood relatives.
それ以前では、母親が直接の相方となっていたようだが、近親相姦のタブーが広まったことから、母方の姉妹、血族以外の女性と変遷したようだ。