How to tell if a play or position is sente
Posted: Fri Dec 05, 2014 10:14 pm
How to tell if a play or position is sente
At some point, beginners get the idea that if the opponent answers a play, that play is sente, and if not, then it is gote. That is one meaning of sente. Note that there is an implicit assumption of locality, since globally, every play but the last (with rare exceptions) is answered.
But after a while it becomes apparent that that idea is unsatisfactory. If Black answers a White play, maybe that is because Black made a mistake, not because the play itself was sente. Black should have played somewhere else. Black’s reply then says more about the player than about the play. That realization leads to a second meaning of sente, as a play that the opponent should answer, whether he actually does or not. As for plays that are answered, we say that they are played with sente, not necessarily that they are sente.
Now we can classify local positions as
1) gote, where the best play by each player is gote;
2) sente, where the best play by one player is sente, while the best play by the opponent is gote; (We call the opponent’s play a reverse sente.)
3) double sente, where the best play by each player is sente.
Obviously, in a double sente position where playing sente gains points by comparison with allowing the opponent to play sente, we should play the double sente immediately -- or soon, anyway. There is even a go proverb that tells us to play double sente early.
These definitions are perfectly sensible. However, whether a position is gote, sente, or double sente still depends upon the rest of the board. How can we tell whether another play is better or worse than a local reply? OC, if we can read the board out, there is no problem. But typically we cannot read the whole go board out. One heuristic that we can use is the size of a play, how much it gains. If the local reply gains more than any other play, it is probably the right play, and the opponent's play is sente. The question then arises of how much a play gains. We’ll get to that question soon.
At some point, beginners get the idea that if the opponent answers a play, that play is sente, and if not, then it is gote. That is one meaning of sente. Note that there is an implicit assumption of locality, since globally, every play but the last (with rare exceptions) is answered.
But after a while it becomes apparent that that idea is unsatisfactory. If Black answers a White play, maybe that is because Black made a mistake, not because the play itself was sente. Black should have played somewhere else. Black’s reply then says more about the player than about the play. That realization leads to a second meaning of sente, as a play that the opponent should answer, whether he actually does or not. As for plays that are answered, we say that they are played with sente, not necessarily that they are sente.
Now we can classify local positions as
1) gote, where the best play by each player is gote;
2) sente, where the best play by one player is sente, while the best play by the opponent is gote; (We call the opponent’s play a reverse sente.)
3) double sente, where the best play by each player is sente.
Obviously, in a double sente position where playing sente gains points by comparison with allowing the opponent to play sente, we should play the double sente immediately -- or soon, anyway. There is even a go proverb that tells us to play double sente early.
These definitions are perfectly sensible. However, whether a position is gote, sente, or double sente still depends upon the rest of the board. How can we tell whether another play is better or worse than a local reply? OC, if we can read the board out, there is no problem. But typically we cannot read the whole go board out. One heuristic that we can use is the size of a play, how much it gains. If the local reply gains more than any other play, it is probably the right play, and the opponent's play is sente. The question then arises of how much a play gains. We’ll get to that question soon.
carries a large threat, larger than it appears to gain. (I say appears to gain because we do not yet know how much it gains.) 
the value of the resulting position is 9 points. The average value is 6.5, (Edit: which we take to be the value of the original position), and the value of a play, assuming that it is gote, is 2.5 points. In that case, 
, also gains 4 points, for a net gain of 0. The reverse sente play moves to a position worth 9 points, and gains only 1 point. (Even though it is the reverse sente that gains points, we call this a 1 point sente.) This asymmetry of the values of the sente and reverse sente is characteristic of local sente. 
is also huge, even though it is not as huge as 