Ippon Shinno (the first-ranked imperial prince) (一品親王)
The Ippon Shinno (the first-ranked imperial prince) was the imperial prince who was granted Ippon, the highest rank (status) under the ritsuryo system (a system of centralized government based on the ritsuryo code). In a broad sense, the Ippon Shinno includes the imperial princess who was granted Ippon (Ippon Naishinno).
For Ippon Shinno, 793,360 m2 of iden (fields given according to the court rank), 800 households of honpu (a vassal household allotted to Ippon) (600 households according to "Shugaisho" [Compendium of fragments, attributed to Kinkata TOIN]) and 160 shijin (lower-rank officers provided to the imperial or noble families and used as a guard or miscellaneous services) were provided. In addition, each one of fumihakase (an official who taught imperial princes classic Confucian writings), Ienokami (butler), Suke (servant), Oimatsurigotohito (大従), Sunaimatsurigotohito (少従), Oisokan (大書吏) and Sunaisokan (少書吏) were sent to Ippon Shinno. His position was the same as Shoichii (Senior First Rank) and Juichii (Junior First Rank), but the treatment he received was higher than for those ranks.
Also he was qualified to assume Daijo-daijin (Grand minister of state) the same as Shoichii and Juichii (But there was no Ippon Shinno who assumed Daijo-daijin during its lifetime.)
The Honi (court rank) shows the hierarchy of Koshin (emperor's family) excluding the emperor and the crown prince, so Ippon Shinno was one of the highest rank in Koshin. However, since the Honi was most of time conferred based on the degree of the intimacy with the emperor, the origin of Koshin's mother, the age, the career and the other social valuation, there was no relation between the Ippon Shinno and the succession to the Imperial Throne. Rather, sometimes Ippon was granted to Koshin who had the right of succession to the Imperial Throne but were excluded as a candidate, as a compensation (ex. the Imperial Prince Tamehira, the Imperial Prince Atsuyasu). But, there were cases Ippon Shinno ascended the throne. The Empress Gensho was conferred Ippon just before the enthronement, and Funado no Okimi (the deposed crown prince) and the Prince Oi (the Emperor Junnin) who were the sons of both Ippon Shinno, the Imperial Prince Niitabe and the Imperial Prince Toneri (both deceased); the Imperial Prince Tokiyasu (the Emperor Koko) was the only Ippon Shinno at that time when he ascended after the Emperor Yozei's de facto dethronement. These were the examples that Ippon Shinno were involved in the succession to the Imperial Throne, but we need to remember that all of them occurred under special circumstances.
The Imperial Prince Hozumi and the Imperial Princess were the first case to be conferred as Ippon in 715. At that time, the abdication of the throne from the Empress Genmei to the Imperial Princess Hitaka had already been determined, and the granting Ippon was to create an environment suitable for the abdication. In order to minimize the discontent in the Imperial Court for granting Ippon to a female Koshin (emperor's family), before male Koshin; it is considered that the Imperial Prince Hozumi, a prominent figure and the eldest among those existing imperial princes of the Emperor Tenmu at that time, was conferred Ippon at the same time. The other three who were conferred Ippon during their lifetime in the Nara period (the Imperial Prince Toneri, the Imperial Prince Niitabe and Taki no Himemiko) were all children of the Emperor Tenmu, and the Imperial Prince Toneri and the Imperial Prince Niitabe served successively at the important posts in the Imperial Court. Since all the children of the Emperor Tenmu, except the Princess Taki who was conferred Ippon as the last existing child of the Emperor Tenmu, were born to mothers who were higher than the rank of the senior court lady; it is supposed that the status of the mothers was deeply related.
As conferring Ippon was socially and financially of a great meaning, the conferment during ones' lifetime was determined very carefully while the conferment after ones' deaths took place relatively frequently; the first Ippon in the Heian period was conferred to the Imperial Prince Kazurawara in 831.
The requirements to be granted Ippon were that the prince must be in nihon (the second court rank for imperial princes) over 20 years through the Nara period to that time, if the period being in nihon was less than 20 year, the prince must be Shikibukyo (Minister of the Ministry of Ceremonial), the highest post for an imperial prince for a long time as well as holding appropriate career as the 'first Imperial Prince.'
In addition, the status of the mother (the possibility to be conferred was low, unless being so-called 'kisakibara' [a child by an empress]) and the degree of intimacy with the emperor and influentials such as the Fujiwara clan were considered. This means that the conferring was made only for the imperial princes who were appropriate, when the capability and achievement as a 'patriarch of a head family' were expected. However, there were imperial princes who fulfilled such requirements but not conferred Ippon, due to financial and political situation at that time. On the other hand, the conferment to imperial princesses became rare, and only the Princess Gishi, sister-uterine of the Emperor Seiwa who was a grandchild of FUJIWARA no Yoshifusa, was conferred Ippon during her lifetime in the ninth century.
During the Sekkanseiji Period (the period of Sekkan [regents and advisers] govern the country), most of the emperor's children except children by empresses demoted from nobility to subject; the requirements for conferring Ippon came to emphasized more on their relationship with the emperor and regents, and less on a career in Nihon.
Also, there were cases such as the Imperial Prince Tamehira and the Imperial Prince Atsuyasu, who had the advantage over the succession to the Imperial Throne, but they had to give up and they were granted Ippon as a compensation (A case of conferring Ippon to the Imperial Prince Kaneakira [MINAMOTO no Kaneakira, former Minister of the Left] can be said as a similar case that he had to return to the Imperial Family because of the Fujiwara clan's strategy.)
The cases of conferring Ippon to particularly imperial princesses, especially grandchildren or great-grandchildren of FUJIWARA no Michinaga increased (Among the children who were conferred between the Emperor Daigo and the Emperor Gosuzaku, four male per nine female which four within were the imperial princesses of the grandchildren or great-grandchildren of Michinaga.)
In this way, the importance of Ippon Shinno rapidly disappeared, and transformed to the treatment allowance to the imperial princes and princesses who were patronized by the emperor or maternal relatives.