The Snuggle and the Tower: The First Two Moves, and How to Answer an Invasion
The most popular first move, the "Snuggle", is a modest move that just slides an edge stone one square diagonally forward. Yet when the AI reads it out, White has almost no correct reply. This article looks at why the Snuggle is strong, the "Tower" you can build from it, and the one way to break that Tower.
Putting your weakest stone to work
In the starting position, the corner stones (1f and 6f for Black) have only one neighbor, so their number is 1. They can move only one square. They are your weakest stones.
The Snuggle slides one of these corner stones to 2e, next to its neighbors.
The moved stone on 2e now has the number 2, and its neighbor 3f went up from 2 to 3. Add up the numbers of all six black stones: 10 in the starting position, 12 after the Snuggle. Of the seven types of first move, the Snuggle is the only one that increases the total.
What about moves that leap forward? Both the Migrating Bird (2f→4d) and the Frontal Breakthrough (3f→3d) drop the total to 6. The stone that jumped is close to being a loner, and the stones left behind lose neighbors. You could say the Snuggle doesn't advance at all; it stores up jumping power.
White can hardly answer correctly
White has 14 possible replies to the Snuggle. When the Strongest AI read them all, the only one that kept the game even was 6a→5b, the same slide on the opposite corner (Shoulder to Shoulder). Shoulder to Shoulder on the same side, 1a→2b, is slightly better for Black. After the other 12, Black is clearly better.
From White's side, this is a position where you want to bring a stone into the center. But the more aggressive the move, the harder it runs into Black's solid shape. The Snuggle wins so often in practice because opponents go wrong here so easily.
When in doubt, bring in the other edge too
Memorizing a different correct second move for Black against each of White's 14 replies is a lot of work. One way around it is to decide that your second move is always to bring in the right-hand corner as well: 6f→5e.
By the old AI's evaluation, this move lost only about 20 points against any White reply, and when White attacked, Black's big lead stayed intact.
Update: later, when we had the new AI play thousands of games against itself, Black's win rate with this shape after White replied with Shoulder to Shoulder stayed around 40%. It is a strong shape that wins 90% against anything else, but against Shoulder to Shoulder another move turns out to win more often. The details are in the Twin Walls article.
Building the Tower
After the Snuggle, Black can form a square block on 2e, 3e, 2f and 3f. This is the Tower. Since all four stones are next to each other, every one of them keeps a number of 3–4 and can jump far. It also serves as a stepping stone when your other stones jump.
The Tower takes as few as 3 moves to build. Of the 6 shortest ways to build it, 5 start with the Snuggle. And since White's stones start on row a, they can't reach Black's home in the two moves Black needs to finish. White cannot stop the Tower.
If they invade, don't move next to the invader
White does have one nasty move, though: the invasion. In the diagram below, White has jumped all the way from 3a to 3d, planting a stone right in front of where the Tower is going to be.
Right now the white stone on 3d has only 2e as a neighbor, so its number is 1. What happens if Black tries to complete the Tower here with 5e→3e?
With 3e now next to 3d, 3d's number goes up to 2. A stone with the number 2 can jump two squares diagonally, and the destination is 1f: the corner Black emptied with the Snuggle, which is White's far row. White gets a stone across. The AI's evaluation was −16.
Black has 13 moves in this position. The 6 that put a stone next to 3d all raise 3d to 2 and let it jump into 1f.
The right move is 4f→1f: fill the empty corner yourself before White can. 4f has three neighbors, 3f, 5f and 5e, so its number is 3, and it can slide three squares sideways into 1f. There is still time to complete the Tower afterward.
- Don't put a stone next to the invader (don't raise its number)
- Fill the corner the Snuggle emptied, first
Stopping you by changing your numbers
There is an invasion even after White replies with Shoulder to Shoulder on the opposite side (6a→5b). Once Black has brought in both edges, White jumps from 4a to 4d.
With the white stone on 4d now next to 5e, 5e's number has changed from 2 to 3. With a 3, 5e can no longer go to 3e. White is stopping the Tower by changing the number of a black stone. When an opponent stone comes close, first check whether your own stones' numbers have changed.
Summary
- Open with the Snuggle: move your weakest stone, the corner stone, next to its neighbors.
- After the Snuggle you can aim for the Tower, and White can't stop it.
- If you are invaded, don't put a stone next to the invader, and fill the empty corner first.
- When an opponent stone approaches, check whether your stones' numbers have changed.
This article is a written version of episode 21 of the YouTube series "bocch定跡研究". The evaluations come from the "Strongest" AI's search and are not fully solved values.