Snowroof



In shogi, Snowroof or Snow Roof Fortress (雁木 gangi, lit. 'goose-wooden') is a Static Rook opening that characteristically uses a Snowroof castle.

It is named after the covered sidewalks (雁木造) connected to buildings in Niigata Prefecture.

Originating in the Edo period, the opening had been a less common one after World War II although it was popular in the prewar era. However, around 2017, the opening has become popular with professional shogi players. The Snowroof structure often appears in computer shogi games.

Characteristics
For Black, this castle positions the king on the 69 square while two golds are at the ears of king (78, 58) and the two silvers are on 67 and 57. The Snowroof castle is strong against attacks directly from above, especially attacks around the central file.

Naitō (1981) notes that the Snowroof was once very popular and had a reputation for being very difficult for a Fortress opponent to defeat. However, by 1981, the Snowroof opening had become less popular because after the Snowroof's opponent trades off their rook pawn, the head of Snowroof player's bishop is vulnerable as it is only defended by an adjacent gold.

Initially, the bishop remains on its starting 88 square. In attack development, this bishop is often moved to the 26 square via three moves passing through B-77 and B-59.

Attacking on the fourth file is common by moving the rook to that file supporting an advancing right silver. However, it is also possible to attack on the third file (Sleeve Rook).

Snowroof game examples
Hisao Hirotsu (Black) used a Snowroof castle against Osamu Katsuura's Fortress position in a 1977 Kisei tournament game on January 11. Here, Hirotsu has completed his Snowroof castle and has moved his king leftward for safety to the 79 square aiming for the 88 square. His bishop has moved to the 59 square (via B-77) and is still defending the 86 square while it may also be developed for attacking. From this 59 square, the bishop may be mobilized on 26 or 37 (since the third pawn has been pushed to 36).

Katsuura (White) has partially completed his Fortress castle, which has been constructed with the Quick Fortress method (which saves one tempo). Since Katsuura's bishop has already been pulled back to 31 on the 31–97 diagonal, he has the possibility of aiming for an eighth file pawn trade plus a bishop trade via ...P-86, Px86 Bx86, Bx86 Rx86 (which actually occurs in the game after this position).

Yoshiharu Habu (White) used a Snowroof castle against Masayuki Toyoshima's Fortress position in a 2014 Asahi Open tournament on February 2.

Habu's Snowroof has been completed. His bishop has been developed completely out of the castle to its attacking position on 84 and his right knight has developed to 73 for attacking as well. Toyoshima constructed a Complete Fortress castle with his right silver on 57 (instead of playing a more common Silver-37 formation) that was subsequently developed into a Fortress Anaguma castle.

Silver Fortress development
One way to improve upon the Snowroof castle's weakness is to develop it into a Silver Fortress.

A Silver Fortress is also structurally more flexible than the typical Gold Fortress.

However, building a Silver Fortress from a Snowroof castle takes several moves. Thus, taking this Snowroof to Silver Fortress approach will probably cause a player to have a strictly defensive position.

Game example
Kinjirō Kimi (木見金治郎) vs Chōtarō Hanada (花田長太郎)

Double Snowroof position

Hanada (White) develops his Snowroof castle into a Silver Fortress castle along with a Sleeve Rook formation.

Right King development
The basic Snowroof formation can transition to a Right King-like structure in which the king and right gold move upward to the fourth file.

Bishop Exchange Declined development
In the 3.B-77 variation of the Bishop Exchange opening, White has a narrow window before making the bishop trade during which they can depart from the opening by closing their bishop diagonal with ...P-44 after it has been opened. After they do so, their position can be developed into a Snowroof strategy. Black's position will likely be some form of Fortress since Black's silver is on the 88 square; however, some professional players have also played other unnamed Static Rook positions as Black.

One reason for playing this move is that White wishes to avoid the large body of theory associated with Bishop Exchange (or Fortress, which also has a large amount of theory behind it) and so chooses a Snowroof form.

Another note about these positions concerns Black's left silver. 4.S-88 is the most common move in Bishop Exchange. However, moving the silver to the 88 square will restrict Black to a Fortress position since the silver cannot be moved to 78 for Left Mino or to 67 for Snowroof. (If Black wishes to keep other options open, then either 4.S-78 or 4.S-68 would have needed to be used instead. 4.S-68 is used for Snowroof while 4.S-78 is the most flexible being compatible with Fortress, Snowroof, and Left Mino.)

If Black wishes to prevent White's Snowroof development from the usual Bishop Exchange opening sequences (that is, 1.P-76 P-84 2.P-26 P-85), then Black will need to use an early 3.P-25 variation.

6.P-25. Once White has declined Bishop Exchange, Black is likely to press forward with a rook pawn aiming to exchange off their second file pawn.

6...B-33. Here White can prevent Black's rook pawn exchange by defending the 24 square with their bishop.

Although 6...G-33 would also prevent the pawn exchange, the gold on 33 is thought to be a bad configuration.

Here White has closed their bishop diagonal (...P-44) and prevented the second file pawn exchange with 6...B-33.

If White is aiming for Snowroof, 6...B-33 is very likely. But, White can also play Snowroof lines that do not stop the pawn exchange, in which case White will play 6...S-42 instead. Then, the pawn exchange 7.P-24 Px24 8.Rx24 is met with 8...S-43, in which the silver protects the pawns on the 34 and 44 squares. However, Yasuhiro Masuda suggests that these are difficult lines and recommends 6...B-33 in spite of the fact that computer shogi engines often play 6...S-42.

7.S-48. Black develops their right offensive silver.

7...S-42. White develops their left silver up to the fourth file for the castle.

8.K-69. Black starts to castle their king leftward. Since their castle will be a Fortress castle (as their silver is on 88), the king cannot move to the 68 square because this would block the bishop from moving to 68, which is necessary to make way for the silver to move 77 while maintaining defense of the 86 square (to prevent White's rook pawn exchange).

8...S-43. With this White completes the basic shape of the Snowroof castle.

After this Black will need to arrange their pieces to construct the Fortress castle and decide upon an attacking strategy. A common idea is to push the fifth file pawn and aim to exchange off the bishops with their bishop moved back to 68. Another is to push the fourth file pawn for a Reclining Silver formation. Since Fortress requires many moves to construct, Black needs to wary that White may attack first. White can develop their pieces naturally and has various attacking options available to them (e.g. using a traditional Snowroof formation with the right silver on 53 possibly paired with a Right Fourth File Rook attack or a Silver Horns structure with the silver on 63 that could shift to Reclining Silver).

Double Snowroof
The Double Snowroof (相雁木 ai-gangi) opening has both players utilizing Snowroof castles.

Old Double Snowroof opening
This is an older opening used in the 20th century up until the end of World War II after which it lost popularity.

Double Snowroof can be developed from an older variant of the Double Wing Attack opening that uses advanced pawns on the central file supported by the right silver.

'''1. P-76 P-34, 2. P-26 P-84, 3. P-25 P-85, 4. G-78 G-32.''' An open bishop diagonal variation of the Double Wing is used as is common of the era. (See Double Wing Attack § Open bishop diagonal variation.)

'''5. P-24 Px24, 6. Rx24.''' Black trades off the second file rook pawns.

'''6...P*23, 7. R-28.''' White fortifies the second file by dropping the pawn back to its original position at the head of bishop. Black retreats their rook to its starting position.

'''7...P-86, 8. Px86 Rx86, 9. P*87 R-82.''' White does the same as Black resulting in a Double Retreating Rook variation of Double Wing.

10. S-48 S-62

11. P-56 P-54

12. K-69 K-41

13. S-57 S-53

14. P-16 P-14

15. P-96 P-94

16. P-36 P-74

17. G-58 G-52

18. P-46 P-64

19. P-66 P-44

20. S-68 S-42

21. S-67 S-43

Silver Horns Snowroof
ツノ銀雁木 tsuno gin gangi

The Silver Horns variant of Snowrook positions the right silver on the 47 square instead of the 57 square as in the traditional Snowroof. This formation allows the possibility of a Reclining Silver development where the right silver moves up to 56.

Silver Horns Snowroof game examples
Keita Kadokura (White) used a Silver Horns Snowroof castle against Shōji Segawa's Fortress position in a 2017 Ōza tournament on October 22.

Silver Horns Snowroof Right King
Like the regular Snowroof formation, there is a Right King variant of the Silver Horns Snowroof (ツノ銀雁木右玉 tsuno gin gangi migi gyoku).

The adjacent diagram shows a game position between Hiromu Watanabe (Black) and Hiroshi Kamiya (White) from August 2016. Kamiya is using a Right King position with his king castled rightward (instead of leftward) on the 62 square near his rook on the eight file and his silvers on the 63 and 43 squares. His bishop has advanced to the 33 square in order to prevent Watanabe from trading off his rook pawn. His opponent Watanabe is using a Fortress castle (via the tempo-saving Quick Fortress method) that has not been fully completed (his left gold remains on its starting 69 square) and his king has still not fully entered the castle.