A samurai could take concubines but their backgrounds were checked by higher-ranked samurai. In many cases, taking a concubine was akin to a marriage. Kidnapping a concubine, although common in fiction, would have been shameful, if not criminal. If the concubine was a commoner, a messenger was sent with betrothal money or a note for exemption of tax to ask for her parents' acceptance. Even though the woman would not be a legal wife, a situation normally considered a demotion, many wealthy merchants believed that being the concubine of a samurai was superior to being the legal wife of a commoner. When a merchant's daughter married a samurai, her family's money erased the samurai's debts, and the samurai's social status improved the standing of the merchant family. If a samurai's commoner concubine gave birth to a son, the son could inherit his father's social status.
Why did merchants prefer that their daughters not marry samurai?
her family's money erased the samurai's debts