In the game of Go
), there are three main stages to the game; fuseki
), chuban (middle game
) and yose
). The difference is not simply how many moves have been made or how many stones are on the goban
(board). There are fundamental differences in the way play progresses during these stages. In general, the chuban starts maybe 10% of the way into the game and finishes maybe 50-70% of the way through, making it about equal in length or perhaps slightly shorter or longer than yose
(depending on the players, their styles, how complicated the fighting has been, etc), but much longer than fuseki
Chuban play is characteristed by the following elements:
During the fuseki, players start out by sketching their spheres of influence and playing out joseki (standard corner patterns). Chuban starts as soon as one player plays inside the other's area of influence, starts a fight, or generally makes any move other than joseki or taking a point in a relatively open area of the goban. This can happen as soon as maybe 15 or 20 moves in the game, or can be maybe twice that, depending on how calmly the players are playing.
Once chuban starts, play begins to focus on weak groups; that is, groups of stones that are close to strong positions of the opponent's, and have not made enough eye space to be safe from attack. Both players will try to weaken the opponent's groups and strengthen their own, and attack weak groups of the opponent in order to try to build territory and/or thickness with which to attack further or build a moyo. At the same time, they will be trying to expand or consolidate their moyos, while waiting for the right opportunity to invade or reduce those of their opponent.
Once there are no longer any weak groups on the board, the game has moved into yose, or endgame. At this point, neither player can launch a severe attack, and play focusses on consolidating territory, expanding one's own, and reducing the opponent's.
A very common criticism of moves made by amateur players, particularly double-digit kyu level players, is that they are playing yose moves in the chuban. Moves that attack and defend weak groups and ones that affect moyos are much larger than anything but the very largest territorial moves. Even a move worth as much as 10 or 15 points of territory should be left until yose, unless it accomplishes something else as well (or is absolute sente). On the other hand, some professional Go players like Sakata Eio have been known to start playing very large yose moves in the late phases of chuban, relying on their uncanny fighting skills to get their weak groups out of trouble when the opponent takes the opportunity to attack. Unless you're as good as Sakata, though (and not many are), it is adviseable to take care of your weak groups before playing moves that do nothing but establish territory. Being able to recognize when chuban is or isn't over, then, is a valuable piece of knowledge.