According to one theory in Osaka, the 'city of merchants,' the belly was split open because it could be cut easier; on the other hand, in Edo, the 'city of samurai,' 'slashing the belly' (harakiri) was considered a taboo, so that the back was slit open.
一説によると、「商人の町」大阪では開きやすい腹を切るが、「武士の町」江戸では「腹を切る」ことを忌み嫌ったために背中から切るのだという。