About the origin of the name 'three lines and a half,' there are opinions that 'three and a half' is a half of 'seven' of the document title 'seven grounds for husbandly repudiation of the wife,' which was one-sidedly issued by the husband in repudiating his wife in uxorilocal marriage as defined in Ritsuryo code during the Nara period, or that 'three and a half' is a half of 'seven,' which is the number of lines customarily written in the permission of marriage issued by the wife's parents.
なお、「三行り半」の名前の由来には、奈良時代の律令に定められた棄妻（婿入婚における、夫からの一方的な離婚。放妻とも言う。）の際に用いられた書状七出之状（しちしゅつのじょう）の「七」を半分に割って三行り半というとする説や、婚姻の際に妻の親元が出す婚姻許可状が7行の文書であることが多かったため、その半分の3行半にするという説などもある。