Asomi (or Ason) (written as 朝臣) was the second rank of kabane (hereditary title) which was created in a system of Yakusa no Kabane (the eight honorary titles) established in 684. Asomi was effectively the highest title amid vassals other than the Imperial Family because Mahito, which was the highest rank, was reserved exclusively for certain imperial clans. Reading it as 'Asomi' is older than the other. It was also written as 阿曽美 or 旦臣 in olden days.
The reason why Asomi was introduced was to create a superior kabane to the others such as Omi, Muraji, Obito, Atai or so on which had long been in existence. Furthermore, the Imperial Family is thought to have tried to seize power by making superior and inferior kabane to create differences in status and favoring certain uji (clan) who have strong loyalty to the Imperial Family.
Asomi was mainly preferentially granted to the clans whose achievement was acknowledged in the Jinshin War and who had the kabane of Omi. The clans with the kabane of Muraji which was the next rank under Omi were given the kabane of Sukune. Later, however, as the clans who contributed to the Imperial Court were granted the kabane of Asomi, most clans in the Nara period had the kabane of Asomi.
As time further passed, most nobles and samurai were descendants of Fujiwara no Asomi, Minamoto no Asomi and Taira no Asomi. Also, with the fall of lower-ranked nobles due to the rise of the warrior class to power, Asomi lost its meaning in the sense of kabane used to rank status, and it became a formality which was used for official documents. The high ranking nobles in the family status were Fujiwara, Minamoto, Taira, Tachibana clans and others in that order.
From the Heian period, a person who was kugyo (sanmi [third court rank] or higher and sangi [councilor]) was called by putting Ason after his uji (clan name) as well as putting a honorific title as ko (minister) or kyo (minister) after his imina (real personal name). A person who was shii (fourth court rank) or higher was called by putting Ason after his uji or imina. Putting Ason after his imina instead of uji refers to Nanori Ason.
e.g. Ashikaga Naidaijin Shonii Minamoto no Ason Yoshimasa Ko
e.g. Hosokawa Ukyo no Daibu Jushiinoge MINAMOTO no Katsumoto Ason
Major clans with the title Asomi
Seiwa-Genji (Minamoto clan): Descendants of the Emperor Seiwa
The clan produced some military leaders such as MINAMOTO no Yoritomo and ASHIKAGA Takauji. Then, out of the longing for a distinguished clan, there were many people such as Ieyasu TOKUGAWA who altered a family tree, making this clan consequently the influential clan for samurai.
Kanmu Heishi (Taira clan): Descendants of the Emperor Kanmu
The clan also had influence after Genji (Minamoto clan). Heike (Taira clan) was well-known. Nobunaga ODA also proclaimed himself to be the title.
The Tachibana clan: The clan did not have as large number as the Minamoto, Taira and Fujiwara clans; however, it was regarded as one of the four influential clans. The clan produced Masashige KUSUNOKI.
The Toyotomi clan: In the Azuchi-Momoyama period, this clan name was granted to kanpaku (chief adviser to the Emperor) Hideyoshi HASHIBA and his family.