Prince Naka no Ōe was made heir apparent and Crown Prince in 661.

661: In the third year of Empress Saimei's reign, she died.
Her son received the succession (senso), but he governed as Crown Prince for the next six years.
 He did not formally accept the monarch's role and duties and powers.

668: Emperor Tenji became the monarch (sokui).
 This was confirmed in ceremonies.

668: In the seventh year of Tenji's reign, flammable water (petroleum) from Echigo Province was presented to Emperor Tenji.

672: Tenji is said to have compiled the first Japanese legal code.
This was the Ōmi Code, consisting of 22 volumes.
 These  law became effective in the last year of Tenji's reign.

Prince Ōtomo (Ōtomo-shinnō) was the favorite son of Emperor Tenji; and he was also the first to be accorded the title of Daijō-daijin.

Tenji improved the military forces which had been established during the Taika reforms.
Prince Naka no Ōe was made heir apparent and Crown Prince in 661.
661: In the third year of Empress Saimei's reign, she died. Her son received the succession (senso), but he governed as Crown Prince for the next six years.  He did not formally accept the monarch's role and duties and powers.
668: Emperor Tenji became the monarch (sokui).  This was confirmed in ceremonies.
668: In the seventh year of Tenji's reign, flammable water (petroleum) from Echigo Province was presented to Emperor Tenji.
672: Tenji is said to have compiled the first Japanese legal code. This was the Ōmi Code, consisting of 22 volumes.  These  law became effective in the last year of Tenji's reign.
Prince Ōtomo (Ōtomo-shinnō) was the favorite son of Emperor Tenji; and he was also the first to be accorded the title of Daijō-daijin.
Tenji improved the military forces which had been established during the Taika reforms.
Emperor Tenji