Sente, A Liberal Interpretation

  Difficulty: Intermediate   Keywords: Theory

This page is a work in progress... I'd like to thank everyone who has reviewed this page. Your comments and corrections have been invaluable. Please keep the feedback coming!

Overview

Sente is a pivotal concept in go. The main Sente page says a player has sente if they "do not have to answer their opponent's last move." This is to be interpreted as not having to answer in a local sense. For example:

[Diagram]

Diagram 1, Is white 1 sente?

Does black need to answer white 1 at 'a'? If not, then black "has sente" and can play in another part of the board with impunity (well, not quite, as we'll see in a moment). If black needs to connect, then we say white 1 was sente. It is common to say both "so-and-so has sente" as well as "move X is sente". But how do you know if black needs to play 'a'? Well, like most things in go, it depends.

When the game is nearly over (when positions on one part of the board don't affect other positions elsewhere on the board) we can be quite precise. In the case of Diagram 1, if black fails to play at 'a', white may play there and kill black's group. That would mean five points for the prisoners plus seven points of territory, for a total of 12 points.

(Aside: if this were to happen, it would cost white two moves to make those twelve points, so white is getting about six points per move on average. This can sometimes be a useful way of thinking about the value of moves.)

Stealing Sente

Now, the key point about sente is, "it's relative". If black can make a move somewhere else that threatens to make more than 12 points, then white really wouldn't have sente with 1 -- black would "steal" sente. Let's look at an example:

[Diagram]

Diagram 2: Black steals sente

What has happened here? White thought black would respond to 1, but black actually found a bigger threat in 2. If white follows up his threat with 3, then black takes 4 and black has gained four points more than white. After 4, white has sente (just like before white played 1, presumably). So actually, by playing things out like this white (whom we assume had sente to begin with) has actually fallen behind by four points! This is a good example of how making threats does not necessarily mean you are gaining points!

But would people really play this way? Not quite. White should not have played 1, but let's suppose, since he is human, that he did. Once black plays 2, white should realize that 2 is a bigger threat than 1 and defend at 4. Then, assuming there are no other large threats for black, black would also go back and defend. Like this:

[Diagram]

Diagram 3: White realizes white 1 not sente.

Now where does the score stand? Black will get two points and white will get two points, so things are even. White's move at 1 has not cost as much as before, but is it a good move? It didn't gain any points in the corner, but it is possible that it has other benefits. If white had stones at the 'a' points, for example, it would seal off some territory at the top.


So at the end of the game, when you can calculate precise values for moves, one can be quite clear about which moves are sente and which are not, who has sente at any given time. (Not that I'm saying it is easy -- see the endgame to see the richness of this area of the game.) But this is only one kind of sente: a purely territorial sente. In my opinion, the concept of sente can be applied very broadly to the entire game right from the beginning. It is not possible to calculate precise numerical values for these cases, but there are, nevertheless, qualitative metrics that can assist making these kinds of judgements. This is where go really becomes interesting...

What are the qualitative metrics? Thickness, Territory, Eyeshape, connectivity, and Efficiency are the main ones. Sente can then be "calculated" by summing together the values of these elements (considering the positive effect on your position minus the negative effect on your opponents position -- a positive). Of course, it's not really so simple, but these pages will attempt to shed some light. At least if you understand these elements you will be playing go instead of just randomly putting stones on the board. In other words, I hope these ideas give you something to think about when you don't know what move to make.

What follows are my definitions for these terms along with examples of how the concepts apply to making value judgements.

Territory

Territory: well, it's the goal of the game, right? CRITICAL ADVICE: don't consider anything as territory until the very end of the game. I'm dead serious. Think about it: you never know what dumb move your opponent might make. Suppose he plays a move like 1 in Diagram 4. You know this isn't the biggest move on the board, you have a larger threat at 2. And you must make that threat. Diagram 4 shows what could happen if you dont. Rather than two points each, white ends up with three to your two plus additional reducing moves at 5/6 and 7-10. (If white played 3 instead of 1, then you could play at 1 and it would be a case of MutualDamage. As it is, white gains from the 1/2 exchange.)

[Diagram]

Diagram 4: Don't count anything as territory

So what should you do? Follow diagram 3, of course. You group might die, but that's okay because you are gaining points overall. That's why you mustn't think the black group in the upper left as territory or even as alive. Look at Diagram 5 to see how this can trip you up.

Territory: Life and Death

[Diagram]

Diagram 5: Why not? Life and death.

It may come to pass that your large, one-eyed group is desperately seeking a way to live and you are faced with the choice in diagram 5. White just played 1 and you need to choose bewteen 'a' and 'b'. If you already considered the upper left corner as being alive, then you might have though it okay to give white the marked wall because you could jump to 'c' to reduce the territory there (see MonkeyJump). Of course, now you see that the corner must die because your one-eyed group is too big to give up.

If black saves the corner with 'b' he gains three points but loses the large one-eyed group, 43 points. Net 40 points to white. Conversely, if he plays 'a' to save the large one-eyed group, he will gain 2 points for the group but lose about 21 points to white. This is the lesser of two evils, so he must give up the corner.

My point is this: you don't know what the future course of the game will be -- in fact, your opponent will try to thwart you at every turn, so don't take anything for granted. Easier said than done, I know.

Territory: Wasted moves

Often, making moves based on territorial concerns will turn out to be wasted, or at least not have full value. I'm sure you've had it happen to you: you make a move to define some territory and your opponent immediately sets about nullifying that very territory. These nullifying moves need to be accounted for when estimating the territorial value of a given move. A lot of the time it is much smaller than you might like to think. Here's an example where a move is played for territorial reasons, but then that reason is later invalidated:

[Diagram]

Diagram 6: Why not? Wasted move.

White just played the marked stone and black has replied with 1. Black seems to think that the upper left side is going to be his territory and 1 is a good, solid move to define the boundary (he might choose 'a', 'b', or 'c' as well, but the point ends up being the same). But the upper left may not become black territory. What if white does this?...

[Diagram]

Diagram 6: Why not? Wasted move.

Now black 1 is not defining territory at all. That doesn't mean it is completely useless -- it is preventing white from making life in the upper left -- but the territorial motives for it no longer apply. This is the point I'm trying to make: when considering what move to play in the opening and middle game, territorial concerns take a back seat to the other issues we will be discussing here.

[Diagram]

Diagram 6: Wasted move. Alternative?

A taste of things to come... It is fairly common for that black 1 to be a good move. Notice that black has a position down the left side. Black 1 works with this to help expand blacks area of influence over the center. Black is open at the top (white could take many points by jumping to a, but the potential for black in the center is better. Helping you to learn how to make these kinds of judgements is a goal of this page.

Territory: Ko

One more example of why you should never consider territory definitely yours: ko. Suppose there is a fairly large ko fight going on. You stand to gain, say, 50 points if you win the fight. Elsewhere you have a small group of eight stones on the second line: technically alive. But your opponent threatens this "live" group as his ko threat. If he kills your group he only stands to gain, say 30 points, so you should definitely ignore the threat, win the ko fight and lose your eight stone group. The point is this: sometimes it is better for you in the overall scheme of things to let your groups die. Somewhat unintuitive, but definitely true.

Territory: Closing Thoughts

The main Sente page also says this: "A move is sente if the value of the follow-up move (when the move is not answered) is larger than the value of the move itself (when the move is answered)." These situations are called priviledged and it is good to recognize them because it can help you estimate where territorial borders will finally stand. Remember to consider that when sacrificing a group, you give up not only the stones and territory they surround, but also their potential for reducing your opponents territory.

Thickness

Thickness: A wall of stones that face out into an open area, generally not yet technically alive but unlikely to die. Their strength comes from the fact that they have many liberties and are well-connected. Enemy groups don't want to get too close because cuts tend to be more likely to succeed in the presence of thickness. Thickness can be very valuable if it isn't nullified by corresponding strong opponent positions. It is also not very useful if it is too close to strong groups of the same color.

Connection

When you keep your opponent separated, you keep his stones weak. One of the hardest judgements in my opinion is knowing when other concerns outweigh keeping your opponent separated. I'd like to look at a few examples of why you might want to let you opponent connect (own weakness too great, larger move elsewhere). I'd also like to look at cases where you should not connect your own stones (larger move elsewhere, indirect defense of peep). A key to this is, of course, understanding the values of moves that are not simply territorial in nature.

Eyeshape

This is the ability for a group to make eyes, and thereby live. Oftentimes it will be possible to make life against the side, but in the process be sealed off from the center. In general, this is a bad idea because a group that is sealed off has almost no effect on the rest of the game. But, of course, that is not always the case. In this section we will look at examples of the relative values of eyeshape and connectivity. I'd also like an example of when it is okay to allow a weak group to develop, and under what conditions that can happen.

Efficiency

Sometimes a single stone stands out because it does so many things simultaneously. If we can find moves that have multiple meanings, we are doing well. To truly excel, our moves should have meaning into the future as well. Like the "wasted move" example under the Territory section, we should strive to keep our stones flexible in the sense that they can be developed in different ways depending on how circumstances progress.


This is a copy of the living page "Sente, A Liberal Interpretation" at Sensei's Library.
(OC) 2005 the Authors, published under the OpenContent License V1.0.
[Welcome to Sensei's Library!]
StartingPoints
ReferenceSection
About