Match Handicap System
Historically in Japan, in longer multi-game matches, it used to be common for professional players of the same rank to alternate colors, rather than use komi, in order to offset the starting advantage. This system also allows for asymmetric assignment of colors (and handicaps), which was used to compensate strength differences:
|same rank||tagai-sen||Players alternate playing with black and white.|
|1 rank (pro)||sen-ai-sen||Weaker player takes black 2 out of every three games (BWB)|
|2 ranks||josen||Weaker player takes black in all games|
|3 ranks||sen-ni-sen||Weaker player always takes black, and takes 2 handicap one out of every three games|
|4 ranks||ni-sen-ni?||Weaker player always takes black, and takes 2 handicap two out of every three games|
|5 ranks||jo-ni?||Weaker player always takes 2 handicap|
|6 ranks||ni-san-ni?||Weaker player takes 2 handicap two out of every three games, and 3 handicap in the other game|
|7 ranks||san-ni-san?||Weaker player takes 3 handicap two out of every three games, and 2 handicap in the other game|
|8 ranks||jo-san?||Weaker player takes 3 handicap in all games|
This system has roughly one stone for every three ranks. In older times, a system with one stone for every two ranks was common (sen for 1 rank difference, sen-ni for 2 ranks, jo-ni? for 3 ranks, etc).
In addition to the rank difference, it was common to change the handicap in a series of games if one player reached a sufficient lead, commonly 4 games, over the other. A player who was forced to take more handicap was said to have been "beaten down", and sometimes longer matches were canceled early to save face for a player who was at risk of being beaten down. A game in which a player was at such risk was known as a kadoban (corner game) for that player.
In games from the GoGoD collection, the custom OH sgf tag is used to denote such handicaps, with parenthesis to indicate which match from the series is in the file. E.g. OH[B(W)B] means that the handicap is sen-ai-sen (BWB) and that this is the game in which the weaker player is playing with white.