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 Post subject: Eye Shape Tier List
Post #1 Posted: Mon Jan 03, 2022 4:52 pm 
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Over on Reddit's /r/baduk, I made a post about Eye Shapes. People didn't like the ASCII diagrams though, so they wanted me to remake the diagrams here.

The original topic: https://www.reddit.com/r/baduk/comments ... tier_list/

I'm roughly a 10k player. So I know I'm not the best, but "tier lists" are a big thing in today's video game communities. In the interest of garnering discussion and maybe attracting some new players to this game, I'll make a "tier list" discussion revolving around "Eye Shape".

For the newer players:

SDK and understanding of Eye Shapes

Building corner / edge life and extending into the center works as a DDK strategy, but its simply not efficient enough to win vs SDK players. You must learn how to make life in the center of the board to compete against SDK.

Making life in the center of the board means that you can make more aggressive cuts, knowing that you can rely upon your life-making skills to save your stones that have been cut-off by your own attack. Alternatively, if your opponent is very aggressive and cross-cutting your stones everywhere, being able to make life out of their relentless attacks will lead to an easy victory! After all, no attack against an "Alive group" can ever possibly succeed.

Eyes progress in roughly 5 steps:

1. Moyo
2. Eye Shape
3. Semi eye (Threaten Eye with Sente)
4. True Eye (Eye even with Gote)
5. Formal Eye

Moyo / Framework -- All games start off with Joseki, building up a loose Moyo/Framework. Expect a few stones to be sacrificed here and there, as your understanding of "big moves" vs "small moves" will be in rapid flux.

Eye Shape -- As your Moyo begins to build strength, they begin to form implicit eye shapes, as well as connections to the eye-shapes you've made. For DDK players, your eye-shapes will mostly be around the corners and sides (where its easier to build eyes), but SDK is really where you must start threatening eyes in the center of the board (a much harder feat to accomplish, but a skill well worth learning)

Semi Eye (True Eye within Sente) -- If you ever gain initiative, you can play one more move to solidify a semi-eye shape into a true-eye. However, there's often too many large moves available in the first 50 moves of the game, so you leave your stones as semi-eyes and chase more important moves elsewhere for a time. However, you often lose Sente/initiative.

True eye (Eye within Gote) -- Once you've decided to spend your Sente on a Semi-Eye, you'll have a true eye. Even if the opponent plays, you are one-move ahead and can progress to a Formal eye before they can catch up with you. As such, the opponent captures Sente now and plays a big move elsewhere. Your opponent may still "catch up" if a Ko occurs however (!!), as Ko allows for two local positions to be played before you respond.

Formal Eye -- In most matches between SDK-level players, the game ends before formal eyes are even made. Both players are good enough at reading can see whether or not a true eye can be turned into a formal eye (or a false-eye), and the battle ends at this point.

Of course, you need a 2nd eye before you reach life (!!). With some careful strategy, you might be able to build two eyes simultaneously with each other. In other situations, maybe you can only build one eye at a time, but connect them together with connecting shapes.



On Eye-stealing moves

In the absence of greater strategy, Eye-stealing moves normally win you 1 point. You turn your opponent's "1" point of territory into a "0". as such, eye-stealing moves are incredibly bad in isolation (!!!!).

That is why eye-shape / semi-eyes / true-eyes can progress. Your opponent can often capture 15, 10, or 5 points with other moves elsewhere. The venerable "Monkey Jump" is often 5+ points for example. As such, an eye-stealing move is rarely useful.

However, if the opponent senses weakness: if a group has been cutoff, then killing an entire group of 15+ stones will lead to a 30-point swing or more!

You should only "eye-steal" if you think you'll benefit from the group's weakness. Stealing an eye may put the group on the run, allowing you to build territory or strength elsewhere on the board. But stealing an eye from an otherwise alive group does almost nothing. There's no benefit to turning a 3-eyed group into a 2-eye group, aside from the 1-point of territory you destroyed.

The smallest formal eye

Click Here To Show Diagram Code
[go]$$c The Smallest Formal Eye
$$.......
$$..XXX..
$$..X.X..
$$..XX...
$$.......[/go]


Many eye-shapes are based on this formal eye-shape. Memorize it, its incredibly important! You need to see and build this shape if you ever hope to build eyes in practice.

The Tiger's Mouth and its Relationship to the Smallest Formal Eye

The Tiger's Mouth is largely a connecting shape, but it does hold some eye-potential because of its relationship to the smallest formal eye shape. Consider this a discussion on how the smallest formal eye is seen as a "threat" on the board.


Click Here To Show Diagram Code
[go]$$c South Tiger mouth
$$.......
$$..XXX..
$$..B.B..
$$..XB....
$$.......[/go]


Click Here To Show Diagram Code
[go]$$c North Tiger mouth
$$.......
$$..XBX..
$$..B.B..
$$..XX....
$$.......[/go]


Click Here To Show Diagram Code
[go]$$c East Tiger mouth
$$.......
$$..XBX..
$$..X.B..
$$..XB....
$$.......[/go]


Click Here To Show Diagram Code
[go]$$c West Tiger mouth
$$.......
$$..XBX..
$$..B.X..
$$..XB....
$$.......[/go]


We can see that the Tiger Mouth serves as a building block, potentially creating the smallest formal eye. While the Tiger Mouth isn't a very strong eye-shape (the opponent can easily destroy the potential-eye with just one move), the Tiger-mouth is highly versatile: serving as BOTH a connection AND an eye-shape simultaneously.

Tier List

Without further ado, here's my tier list for eye shapes. This tier list does NOT think about connecting shapes or attacking shapes. This is "purely" a focus on eye-shapes and living space and efficiency of the form.

S-Tier: Ponnuki
A-Tier: Mouth, Table
B-Tier: Stretch+One Point Jump, Lion's Mouth,
C-Tier: Dog's Head, Tiger Mouth
D-Tier: Tower

As I stated before, this is just my humble, 10k opinion on the eye-potential of these various shapes. I'll list off my reasoning now.

Initial Tier Discussion

Click Here To Show Diagram Code
[go]$$c Ponnuki
$$.....
$$..X..
$$.XWX.
$$..X..
$$.....[/go]


A formal Ponnuki means capturing an enemy stone in the center. But you'll find that the diamond-shape (without the capture) is still very efficient at building eyes, especially in the corners. With a capture, you've spent 4 turns while the opponent has spent 1 turn locally, which is the king of efficiency.

Without the opponent's sacrifice at "o", the Ponnuki is a diamond shape and is far less efficient (Your 4 turns vs Zero turns from the opponent).

Looking ahead to the rest of the game: "x" only needs need 3 more plays to make the formal eye shape, while "o" needs 2 plays to convert the Ponnuki into a false eye.

The Ponnuki almost never appears by itself however. A common 3-3 Joseki ends with the following position:


Click Here To Show Diagram Code
[go]$$ 3-3 Invasion into double-hane Joseki
$$---------|
$$..aXb....|
$$..XWXOO..|
$$..cXOXX..|
$$....OOO..|[/go]


Here, we can see that Black has created a Ponnuki, killing the circled-stone and radiating strength towards the left.

Now consider the Smallest Formal Eye shape: black needs to play at a, b, and c before this is a formal eye. But a and b are both self-atari moves by white! As such, Black doesn't need to play at a or b yet. Furthermore, "a" is already looking like an eye-shape itself! This ponnuki is a semi-eye as it stands, and only about 3 moves away from achieving formal-life (two formal eyes)

"c" is the only move that can kill the formal-eye. If white attacks at c and survives, then the Ponnuki is converted into a false eye. However, this is a terrible move for white.


Click Here To Show Diagram Code
[go]$$ Terrible attempt at White invading
$$---------|
$$...X.....|
$$..X.XOO..|
$$..OXOXX..|
$$...XOOO..|[/go]


Black can cut the stone at "c", and now white has to spend a large amount of effort trying to keep the invading stone alive. Black almost certainly will benefit from such a terrible move by white.

As such, this Ponnuki has already progressed into a Semi-Eye: that is, Black only needs to play at one spot to make an Eye (even within Gote).

Click Here To Show Diagram Code
[go]$$ Black plays to trade Sente to progress from Semi-eye into True-eye
$$---------|
$$...X.....|
$$..X.XOO..|
$$..1XOXX..|
$$....OOO..|[/go]


Sente is extremely powerful in the early and mid-games however, so building this true-eye is not worthwhile. In practice, this ponnuki semi-eye sits around for 30, 40, 50 or more turns, while the players do other things elsewhere in the board.

Given the extreme efficiency of the Ponnuki, it is the only "S-Tier" eye-shape I can think of. Other shapes are great at making eyes, but nothing is quite as good as a Ponnuki.


--------------

Click Here To Show Diagram Code
[go]$$ Mouth
$$.......
$$.XaX...
$$.XWb...
$$..XX...
$$.......[/go]


In most circumstances, the Mouth shape is a semi-eye as it stands. As such, its a bit heavy, requiring 5 moves to create. But if the opponent tried to cut your group (white's circled stone), you'll find that creating this semi-eye is often a very good move.

White requires a lot of local strength before it can hope to play the eye-ruining attacks at a or b. As such, this shape is almost certainly a semi-eye, unless you're deeply surrounded by white stones.

But with 5-moves required to make the Mouth shape, its very heavy and inefficient at connecting up. If your opponent doesn't play the marked stone location, you'll find that playing this heavy often loses Sente / initiative. As such, the Mouth Shape is A-tier, very good at making eyes, but its efficiency knocks it a point down compared to the Ponnuki.



Click Here To Show Diagram Code
[go]$$ Table
$$.......
$$.X.X...
$$.X.....
$$...X...
$$.......[/go]



As a "child" of the Stretch + one-point jump / Ikken-tobi from a Stretch, the Table is a very common and useful pattern that results from common joseki.

The easiest eye-stealing move of the Table is:

Click Here To Show Diagram Code
[go]$$W Poor attempt at Eye-stealing from the Table
$$.XaX...
$$.X2b...
$$..1X...
$$..c....[/go]


Getting any of the edges ruins the smallest formal eye shape, but black can respond at 2, and now you have a Lion-mouth shape + simultaneously threatening the 1-stone with a ladder (played at c). As is discussed with the Lion-mouth shape later, the a and b locations are threatening to create two eyes now!

As such, attacking the Table and removing the eye is difficult, earning this shape a place in A-tier. This "Table" is closely related to the Bamboo joint, consider a Table if you need eye-space, while the Bamboo-joint is used if you need a stronger connection.


Click Here To Show Diagram Code
[go]$$ Stretch + One-point Jump (Ikken-tobi from a Stretch)
$$........
$$..X.X...
$$..X.....
$$........[/go]


This is a very common shape. By itself, it plays more like a connection rather than an eye-space, but you can build it out into the "Table" shape or Bamboo joint as needed. Though only B-tier in terms of Eye-shape, this is a very strong connecting shape, meaning it has huge flexibility. Its furthermore "light", meaning you can sacrifice either of the two sets of stones in an emergency.

This shape occurs naturally with almost any knights-approach. Consider the following Joseki pattern:


Click Here To Show Diagram Code
[go]$$ Knight's approach Joseki
$$........
$$...21.5.
$$..O.3...
$$........[/go]


As the "prototype" to so many other important shapes, this shape threatens to make a Table (thus turning into an Eye), or a Bamboo-joint (threatening one of the strongest connection shapes in the game). There are many other shapes here as well.

But as it stands, I rate this shape B-tier in terms of making eye space. Its not hard at all for the opponent to ruin the eye here, you need another 1 or 2 moves to make something resembling the smallest formal eye. Still, its flexibility means you have the ability to "move the eye" where you need it, depending on how the opponent attacks you.

Click Here To Show Diagram Code
[go]$$ Lion's Mouth
$$.......
$$.XaX...
$$.bX....
$$.X.....
$$.......[/go]


The Lion's Mouth is two Tiger Mouths back-to-back. It occurs frequently in pushing battles when both players push diagonally against each other, or when a double-hane is resolved peacefully. There are two prototype-eyes being made, at a and b.

With weakness, the opponent can turn the Lion's Mouth into a B2-bomber, which is one of the worst-shapes ever.

Click Here To Show Diagram Code
[go]$$W White attacks Lion's mouth into a B2 Bomber
$$........
$$....1...
$$..X2XO..
$$..4XO...
$$.3XO.O..
$$..O.....[/go]


As such, the Lion's Mouth is a very volatile shape. It might be two eyes, or it might be a dumpling if pushed into a bad situation. With some defense, it threatens two eyes and is close to life, but its a "sharp" shape, a misplay could turn it into a B2 Bomber instead, and you'll be on the run.

Click Here To Show Diagram Code
[go]$$ Better defended Lion's Mouth
$$........
$$...aXO..
$$..X.XO..
$$.b.XO...
$$.XXO.O..
$$..OO....[/go]


In this case, both White and Black are close to achieving life, with two prototype-eyes. The points at a and b remain black's weakpoints, so even with some additional defense stones, this shape is not quite settled yet!

But since this appears in practically any diagonal-pushing battle, you should practice this position and learn to defend it well!


Click Here To Show Diagram Code
[go]$$ Dog's Head
$$........
$$..X.....
$$...aX...
$$..X.....
$$........[/go]


The one-point jump is pretty common. Making a jump from the center of a one-point jump is a natural way to convert a one-point jump into a potential eye-space. However, whenever I play this myself, it has felt more like a connection rather than an eye.

Yes, the "a" point is a potential eye here, but its also a potential invasion spot by white. As such, I rate this a C-tier eye shape. You're beginning to make an eye, but you need another turn or two before its really a formal eye.

This is a very good shape though: you can sacrifice any of the three stones if any emergency comes up, you can easily connect up at "a" if you need a connection, or you can build out an eye if that's what you need. There's a supreme amount of flexibility here.

Click Here To Show Diagram Code
[go]$$ Tiger Mouth
$$..b....
$$.XaX...
$$..XO...[/go]


The Tiger Mouth is a common connecting pattern. But unlike the formal connection, the Tiger Mouth leaves a space at "a" that might form into an eye in the future!

It is this "eye-potential" that makes the Tiger Mouth useful over a formal connection. Still, its a very weak eye. White can simply play at "b", which kills the eye and even threatens to sever the connection.

Click Here To Show Diagram Code
[go]$$ Tower
$$.......
$$.X.X...
$$..ab...
$$.Xcd...
$$.......[/go]


Tower is a good shape, but its a weak "eye shape". With attacking points at a, b, c and d... the chance of turning a Tower into an eye is rather small. The main benefit to the Tower shape is sacrifice: you can cut off almost any stone and end up in a good position. But if your opponent is closing into your position and you need eye space, stay away from Tower IMO.

Despite being in "D-tier" for eye-shape, I think the Tower connection is still very powerful and flexible. With a play at "a", you instantly get the Lion's Mouth. With a play at "b" or "c", you get the table shape!

As such, you're one turn away from either a strong connection, or a strong eye-potential. Leaving the choice up to the future (depending on how the board plays out) is extremely useful indeed! Studying the Tower, and its "children" (table and Lion's Mouth, and even the 4-corner shape) is recommended, even if this shape is kinda bad as it stands for eyes.


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 Post subject: Re: Eye Shape Tier List
Post #2 Posted: Tue Jan 04, 2022 3:52 pm 
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Click Here To Show Diagram Code
[go]$$ Bamboo Joint
$$...d...
$$..XaX..
$$..XbX..
$$...c...[/go]


I rate the Bamboo-Joint F-tier for eyes. While this is one of the strongest connection shapes in the game, its pretty awful at making eye-space. The a and b points are miai, the connection cannot be severed in typical circumstances because if white plays "a", then black plays "b". And if white plays "b", black plays "a".

However, there's no miai point for eye-space! c and d are the spots white will play at to try to stop an eye, and things will likely progress in the following manner.

Click Here To Show Diagram Code
[go]$$W Bamboo Joint loses its eyes
$$...1...
$$..X2X..
$$..X.X..
$$...3...[/go]


As a result, black is left with no eyes at all. Despite being a strong connection, this shape is utterly terrible at actually making eyes. After this sequence, the 1 and 3 white-stones aren't threatened in the slightest, and can likely run away safely.

If eye-space is important, build the closely-related "Table" shape instead. Bamboo joint focuses almost entirely on connections, sacrificing eye potential.

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 Post subject: Re: Eye Shape Tier List
Post #3 Posted: Tue Jan 04, 2022 4:31 pm 
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Pretty good and thorough review of shapes. A few points.

Consider
Click Here To Show Diagram Code
[go]$$c The Smallest Formal Eye?
$$------
$$|.X..
$$|X.X.
$$|XXX.
$$|....[/go]


This is a two-headed dragon that is already pass-alive (no matter how many times W plays, W cannot kill). Similarly, diagonal moves can greatly help life/liberties in a capture race due to the effect you mentioned of defending the other two adjacent 2x2 points simultaneously by shorting their liberties, especially in combination with other diagonal moves. So it doesn't even have to be a formal eye to be very helpful.

Some analysis of opponent positions might be helpful to make this analysis more advanced. For example,

You generally only want to play "good shape" if you can't make more efficient shape by leaning on your opponent's weaknesses nearby. This means that each of these should be threatened on at least two or three sides to be good. There is a very rarely mentioned concept of "weakness balance" between opposing shapes that you might want to watch out for. For example if you can make a tiger's mouth shape in (semi)-sente by attacking, then playing the Dog's head will be overconcentrated, and the large dog's head (is it called the horse's head?) is preferable.

Click Here To Show Diagram Code
[go]$$ Mouth
$$.c.....
$$.XaX...
$$.XWb...
$$..XXd..
$$.......[/go]


Mouth shape:
Quote:
As such, its a bit heavy, requiring 5 moves to create.
In your diagram, B has 5 stones, W has 1 dead stone. Perhaps you should say B has 4 more stones (unless the W stone isn't there). This mouth shape may be worse than a ponnuki, but it is more of a fighting shape if you have a slight advantage locally. (I think a ponnuki is more often used in combination with strong influence several spaces away or just tenuki to let the opponent attack it). I would say the mouth shape is spoilt (or becomes rotten) if W can force you to play a or b with an atari on your enclosing stones at c or d. But if this isn't the case, then the shape is not only resilient but has good influence.

Dog's head: This is most commonly played when the opponent is very thick on both sides, but this runs towards empty space or threatens another eye locally (without needing to go near thickness. The point is to cover the weak point at a more severely (so that W attacking at a has less liberties), so another move will very likely make a or one of its neighbours into an eye.

Click Here To Show Diagram Code
[go]$$ Better defended Lion's Mouth
$$....c...
$$...aXO..
$$..X.XO..
$$.b.XO...
$$.XXO.O..
$$.dOO....[/go]


When I see this shape, I think B is getting pushed around. Although the shape is close to symmetric, W seems to have a slight advantage in pushing for space at the boundaries. It feels like B has played too close to W's thick tiger mouth shape and should have just played nobi earlier. In terms of eyespace, B reinforcing by extending at say c or d is powerful against W's shape (as well as space development), so it is difficult to attack at a/b just yet.

Click Here To Show Diagram Code
[go]$$ Tiger Mouth
$$..b....
$$cXaX...
$$.dXO...[/go]


The tiger's mouth is often best (compared to being solid at a) if the extra influence at c (to weaken W's chances there) or d (at the mutual boundary point) is valuable. Sometimes W's aji of b can simply be answered by a and this might not be a good exchange for W since B's tiger mouth stone goes from 3 liberties to 6 by connecting and could have been attacked from the other side.

Tower shape: You make good points. This is more common than the dog's head in a running fight, because your opponent likely has weaknesses. Although W can poke at a, such a stone is very weak itself, and B's game plan is to say that while the poke is annoying (so it isn't easy to make a clean eye (never mind 2) from a any more, but such a move makes B thick in aji/liberties, so B can counterattack W on the other side more severely.

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 Post subject: Re: Eye Shape Tier List
Post #4 Posted: Wed Jan 05, 2022 12:15 am 
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dhu163 wrote:
Consider
Click Here To Show Diagram Code
[go]$$c The Smallest Formal Eye?
$$------
$$|.X..
$$|X.X.
$$|XXX.
$$|....[/go]


This is a two-headed dragon that is already pass-alive


Perhaps I focused too much on the single-eye shape. The double-eye shapes are certainly more efficient: "corner 6 life", "side 8 life", and "center 10 life". (see: https://senseis.xmp.net/?SmallestGroupWithTwoEyes).

I think I made this omission because even as a lower-ranked DDK, I knew that memorizing all simple-life shapes was important. But it didn't occur to me that I should memorize the single-eye shape until recently. Now that I've learned the single-eye shape, I'm able to much more quickly figure out "false eyes" from "true eyes" (aka: 3-corners taken up. If the opponent takes 2-corners, then its a false eye.).

Yes, make double-eyes if possible. But if those aren't possible think of ways to make single-eyes (perhaps two separate groups of single eyes).

Quote:
Similarly, diagonal moves can greatly help life/liberties in a capture race due to the effect you mentioned of defending the other two adjacent 2x2 points simultaneously by shorting their liberties, especially in combination with other diagonal moves. So it doesn't even have to be a formal eye to be very helpful.


That's a good point. All of the simple life shapes: corner-6, side-8, and center-10 are "diagonal eyes". The threat of a double-eye in a few short moves is significant when you have a few diagonal moves here or there. Perhaps its just my skill level though, but I haven't really been able to reliably convert those diagonal positions into life quite yet. Perhaps I need more practice with this concept. Are there any shapes you seek when trying to make dual-eyes in one go? I kind-of see it as possible from the Lion's Mouth or maybe the Table, but its harder for me to see it in other shapes.

Quote:
You generally only want to play "good shape" if you can't make more efficient shape by leaning on your opponent's weaknesses nearby. This means that each of these should be threatened on at least two or three sides to be good. There is a very rarely mentioned concept of "weakness balance" between opposing shapes that you might want to watch out for. For example if you can make a tiger's mouth shape in (semi)-sente by attacking, then playing the Dog's head will be overconcentrated, and the large dog's head (is it called the horse's head?) is preferable.


I do realize that Go is definitely a game where context is king. As you've mentioned: thick shape is needed if the opponent is thick, while thinner, faster shapes are important when the opponent is thin. Ensuring a proper balance of power is the key to a successful attack and/or defense. If you're too thick, the opponent runs away before you deliver serious damage. If too thin, your opponent will cut your groups up into shreds and you'll find life outside of your reach.

Still, there's a tradition inside of video game circles to simplify context for the purpose of tier-lists, but then to "add the context back in" during tier-list discussions. Ideally, you don't want to bog down the readers with too many details, and I may have already err'd on the side of too many words as it is.

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 Post subject: Re: Eye Shape Tier List
Post #5 Posted: Fri Jan 07, 2022 1:32 pm 
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I'm having difficulty placing the Tortoise Shell between A tier or S tier. I'm inclined for it to be A-tier.

Click Here To Show Diagram Code
[go]$$ Tortoise Shell
$$ .....
$$ ..X..
$$ .XWX.
$$ .XWX.
$$ ..X..
$$ .....[/go]


The Tortoise Shell occurs when 2-stones are captured. Black has spent 6-stones for the arrangement, while white has spent 2-stones / turns, meaning you have ~4-turns to build a Tortoise shell. As a "capture shape", it shares a lot of similarities with the Ponnuki, but I ultimately find the Tortoise Shell less a specific thing to aim for and more of a shape that just "happens". Its somewhat rare for the opponent to sacrifice two stones out of the blue, and when opponents decide to sacrifice two stones, they almost certainly are getting some kind of benefit from it.

As a specific shape, you're still 6 turns vs 2 turns, meaning its still slower than a Ponnuki, but its certainly more stable and easier to convert into a pass-eye / formal-eye shape.

Click Here To Show Diagram Code
[go]$$ Tortoise Shell
$$ .....
$$ .aXb.
$$ .XcX.
$$ .XdX.
$$ .eXf.
$$ .....[/go]


Like a Ponnuki, you require additional moves to turn the Tortoise shell into a pass-eye / formal-eye shape. The smallest possibility is 2 moves: a+d for example, meaning you're closer to life than a Ponnuki (which requires 3-moves).

Click Here To Show Diagram Code
[go]$$ Tortoise Shell a+d formal eye
$$ .....
$$ .BX..
$$ .X.X.
$$ .XBX.
$$ ..X..
$$ .....[/go]


There are additional possibilities, some with 3 moves.

Click Here To Show Diagram Code
[go]$$ Tortoise Shell a+e+f formal eye
$$ .....
$$ .BX..
$$ .X.X.
$$ .X.X.
$$ .BXB.
$$ .....[/go]


In most cases, your Tortoise Shell will occur in an area with extreme fighting.

Click Here To Show Diagram Code
[go]$$ Tortoise Shell in a fight
$$ .OOO..
$$ .OXO..
$$ .X.X..
$$ .X.X..
$$ ..X...
$$ .....[/go]


Something like this for example. With two same-side corners captured, you must be careful about the opponent's ability to turn the entire formation into a dumpling shape.

Click Here To Show Diagram Code
[go]$$W Tortoise Shell converted into Dumpling
$$ .OOO..
$$ .OXO..
$$ .X1X..
$$ .X2X..
$$ ..X...
$$ .....[/go]


Under these circumstances, the eye is completely stolen. The "a" and "b" spaces (or e+f) spaces are therefore the "weak points' of the Tortoise shell, and you need to be careful to take those points during a tight fight if you want to reliably form an eye. If both "a" and "b" are taken, you still have a chance at an eye by taking the "c" point. But if the opponent plays a+b+c first, the entire shape collapses into a false eye.

--------

The importance of a+b+c as a combined weakness is more clear in more realistic scenarios.

Click Here To Show Diagram Code
[go]$$W Complicated Tortoise Shell converted into False-eye
$$ .OOO..
$$ .OX...
$$ .OX...
$$ .OXO..
$$ .WXW..
$$ .X1X..
$$ .X2X..
$$ ..X...
$$ .....[/go]


Notice that the circled stones have captured the "a"+"b" points, and fighting has broken out in the north. Black cannot build a 2nd eye with its northern stones. The final key point of "c" (or the spot that white attacks with 1) is the difference between a false-eye and a true-eye.

Click Here To Show Diagram Code
[go]$$B Black defends in time, keeping one eye
$$ .OOO..
$$ .OX...
$$ .OX...
$$ .OXO..
$$ .WXW..
$$ .X1X..
$$ .X.X..
$$ ..X...
$$ .....[/go]


Upon realizing that there's no hope for a 2nd eye with the northern attack, Black should capture the key point to keep at least one eye built, and then maybe hope for more luck in the south.


---------

The Tortoise Shell may naturally occur when playing with the Tiger Mouth shape.

Click Here To Show Diagram Code
[go]$$ Tiger Mouth inside of Tortoise Shell
$$ .....
$$ ..B..
$$ .B.B.
$$ .X.X.
$$ ..X..
$$ .....[/go]


Its also somewhat related to the Stretch + One-point Jump.

Click Here To Show Diagram Code
[go]$$ Stretch + One-point Jump inside of Tortoise Shell
$$ .....
$$ ..X..
$$ .B.B.
$$ .B.X.
$$ ..X..
$$ .....[/go]


---------

Even in the most ideal of situations, the Tortoise Shell is an exceptionally heavy shape requiring 6-stones. Like the "Mouth" shape, it only really sees use in the heaviest of fighting situations. In isolation, the Tortoise Shell is a true eye, but you'll find that the shape only occurs naturally in areas with lots of fighting (and thus its position in practice will be somewhere between eye-shape to semi-eye, depending on how many stones your opponents have nearby).

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