Yobai (“night crawling’) was an ancient Japanese mating custom, wherein men were allowed to sneak into the rooms of young unmarried women at night.

“According to ethnologist Akamatsu Keisuke, the practice varied from place to place. In some places, any post-puberty woman, married or unmarried, could be visited by any post-puberty man, married or unmarried, from the village and even by men from other villages and travellers. In other places, only married women and widows could be visited, while single girls could not. And there were variations; for example, the "closed type" yobai was a custom in which only men from the same village had the right of visitation.”