Go

``Go is an ancient game that originated in China, with a definite history of over 3000 years, although there are historians who say that the game was invented more than 4000 years ago. In this game, each player tries to exert more influence on territory than her opponent, using threats of death, capture, or isolation. It is, therefore, a symbolic representation of the relationships between nations. Go is getting increasingly popular around the world, especially in Asian, European and American countries, with many worldwide competitions being held.'' (From http://senseis.xmp.net/)

Two players, black and white, alternately place stones on the empty points of the board. Players may also pass. The standard board is 19 by 19 (i.e. the board has 19 lines by 19 lines), but 13-by-13 and 9-by-9 boards can also be used. The game starts with an empty board and ends when it is divided into black and white areas. The one who has a larger area wins. (See Figure 1)

Stones of one colour form a block when they are 4-connected. Empty points that are 4-connected to a block are called its liberties. When a block loses its last liberty, it is removed from the board. (See Figure 1) After each move, surrounded opponent blocks are removed and only after that, it is checked whether the block of the played move has liberties or not.

There are different rulesets that define more carefully what a ``larger area'' is, whether suicide is legal or not, and how infinite repetitions are forbidden. For a more detailed description with examples, see
http://www.britgo.org/intro/intro2.html

Figure 1: The rules of Go. Left: The end of a game. Black has won, because he controls a greater area on the board. The lone white stone in the top is assumed dead. Right: White has three blocks and black two. White can capture the block of 8 black stones by playing in the middle and thus removing their last liberty. Captured stones are removed from the board.