Sen-ai-sen

Professional go handicaps were a system developed in Japan, in the Edo period, for handicapping professional players of the game of go against each other. With the abolition of the Oteai system, which from the 1920s had used some handicap games to determine the go ranking of professional players, this system has become obsolete. It is now completely superseded by the use of komidashi. Knowledge of it is required to understand the conditions of play in historical go matches, particularly the jubango that died out around 1960.

The professional go ranks have traditionally been divided into nine levels, with shodan or 1 dan being the initial grade for a student player certified as professional (kishi in Japanese). The ranks go up to 9 dan, the whole system being based on old customs from the Chinese Imperial court. (The imperial court had 1 pin as highest and 9 pin as lowest however) To this day there are nine professional dan levels in China and South Korean as well as in Japan; the same applies in Taiwan. The basic system described at go handicaps is insufficient to provide an accurate ranking, because professional levels are closer together. It is considered inconceivable that any pro should take a four-stone handicap from another.

Theoretically all nine pro levels were within a three-stone handicap spread. In modern times that has become even tighter, since professional shodan is not generally awarded to players who will remain at that level thereafter. In terms of a notional Elo system, if 100 points is one amateur rank, the professional levels were notionally more like 35 points apart. That in contemporary go and under modern playing conditions has been compressed down, to fewer than 30 points apart.

In the Edo period, however, apprentice professionals would often be admitted as professional 1 dan at a young age (Honinbo Shusai at 13, but this was by no means young; Takagawa Kaku was admitted at 11). These players might be at current amateur 6 dan or even 5 dan level, but would be promising talents.

...
Wikipedia