Kaō first appeared in China during the Tang Dynasty, and began to be used in Japan during the Heian period. Though their use became far less widespread after the Edo period, they continue to be used even by some contemporary politicians and other famous people. The reading and identification of individual kaō often requires specialist knowledge; whole books devoted to the topic have been published.
Often used by Japanese swordsmith on sword tang (nakago).
^上島 有 [Tamotsu Kamishima] (December 2004), 中世花押の謎を解く―足利将軍家とその花押 [Cracking the Riddle of Kaō from the Middle Ages: The Ashikaga Shogunate and their Kaō], 山川出版社 [Yamakawa Shuppansha], ISBN978-4-634-52330-2, NDL20717189