Isshi Sōden (一子相伝 Sole Succession?)
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 Shinken tradition, as 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 renounce the art or suffer the consequences - which include having their memory wiped or their fists destroyed.
The Nanto Hōō Ken school also follows this same tradition. However, in this case, the successor must also kill his master in order to complete his training.