Isshi Soden

Isshi Sōden (一子相伝) is a Japanese term which decrees that a father's trade secrets can only be passed down to one son and heir. In Hokuto no Ken, this is the essence of the Hokuto Shin Ken tradition, where there can be only one successor for each generation. The successor will then inherit the hidden secrets of the style, whilst those who failed to become successor must give up the style or face having their fists destroyed or memories erased.

The Nanto Hōō Ken school also follows this same tradition, however, in this case the successor must also kill his master in order to surpass him.