Twin Walls: A Strategy That Wins 90%, and Its One Weakness
Snuggle on move one, then bring in the other edge on move two. Bringing both edges inward is "Twin Walls". In about 3,700 games between the new AIs, Twin Walls won over 90% against almost every reply. But there is exactly one defense it can't beat.
The Twin Walls shape
Black's first move is the Snuggle (1f→2e), sliding the left edge stone one square diagonally forward. After White replies, Black's second move slides the right edge stone in the same way (6f→5e).
Left where they are, edge stones have only one neighbor and do almost nothing. Bringing both of them inward early connects all six stones to their neighbors.
Why it is strong
White has 14 possible first moves against the Snuggle. Against 12 of them, Black's win rate with Twin Walls averaged 91%. Even against the toughest reply (3a→3c) it was 84%, and against the weakest (3a→5c) it was 96%.
Even with the Strongest AI reading deeply, Twin Walls was Black's best second move in 11 of those 12 cases. It is one of the most natural, and strongest, shapes in the bocch opening.
The weakness: Shoulder to Shoulder
It fails when White's first move is Shoulder to Shoulder. That is White sliding an edge stone one square diagonally forward, the same shape as Black's Snuggle.
If White plays Shoulder to Shoulder on the same side as Black (1a→2b) and Black plays Twin Walls, Black's win rate is 43%. On the opposite side (6a→5b) it drops to 37%.
White's answer is simple: if Black plays Twin Walls, White brings in its remaining edge too and makes the same shape. With the Strongest AI reading 15 moves ahead, the position is nearly even. Yet when the AIs actually play it out, White wins more often. Even if it is equal with perfect reading, it is a position that is harder for Black to keep playing well.
In episode 24 of the video series "bocch定跡研究", I said Twin Walls against Shoulder to Shoulder wins 15%. Measuring again with the conditions separated gave 37–43%. It is certainly difficult, but 15% was an overstatement.
If they play Shoulder to Shoulder, drop Twin Walls
Against same-side Shoulder to Shoulder, push the center stone three rows
If White plays Shoulder to Shoulder on the same side as Black, instead of Twin Walls, push the center stone on 3f three rows in one go (3f→3c). Thanks to the Snuggle, 3f has three neighbors, so it can go straight three rows.
This move won 63%, 20 points higher than Twin Walls' 43%. Shield & Spear (2f→2d) won 48%.
Against opposite-side Shoulder to Shoulder, Escort Bird or Shield & Spear
If White plays Shoulder to Shoulder on the opposite side, advance the stone next to the Snuggle. Flying it diagonally with the Escort Bird (2f→4d) won 59%, and pushing it straight with Shield & Spear (2f→2d) also won 59%. Both are far better than Twin Walls' 37%.
| White's 1st move | Black's 2nd move | Black's win rate |
|---|---|---|
| Anything but Shoulder to Shoulder (12 moves) | Twin Walls 6f→5e | 91% |
| Same-side Shoulder to Shoulder 1a→2b | Three-row push 3f→3c | 63% |
| Opposite-side Shoulder to Shoulder 6a→5b | Escort Bird 2f→4d / Shield & Spear 2f→2d | 59% |
From White's side
From White's point of view, it is the other way around. If Black opens with the Snuggle, White's only first move is Shoulder to Shoulder. With anything else, Black plays Twin Walls and White wins only about 10% on average.
With Shoulder to Shoulder in place, if Black plays Twin Walls anyway, White just brings in its other edge in reply and actually becomes the side more likely to win.
Summary
- After the Snuggle, play Twin Walls unless the opponent played Shoulder to Shoulder. It wins 90%.
- If the opponent played Shoulder to Shoulder, drop Twin Walls: 3f→3c on the same side, Escort Bird or Shield & Spear on the opposite side.
- As White, always answer the Snuggle with Shoulder to Shoulder. If Twin Walls comes, bring in your other edge too.
Where the numbers come from: games between Strongest CPUs. After the fixed move sequence, the next 2 moves were chosen at random from the top 3 moves, and after that each move was played with 120 milliseconds of thinking. Win rates count draws as half. The 12 replies other than Shoulder to Shoulder total 1,190 games for Twin Walls; the Shoulder to Shoulder positions have 200–500 games per move.