Double Wing Attack

In shogi, Double Wing Attack or simply Wing Attack or Centre Game (相掛かり or 相懸り "aigakari") is a Double Static Rook opening in which both sides directly advance their rook pawns forward on the second and eighth files toward their opponent's bishop often with the first several moves on each side being identical or very similar.
History and general discussion.
The Double Wing Attack is one of the most traditional openings dating back to over 250 years ago.
Double Wing openings have two general variations. The first one has both bishop diagonals open (that is, P-76 and P-34) while the second type has bishop diagonals closed. The open bishop diagonal variations were most common historically being played from the 1700s and throughout the first part of the twentieth century. 
However, in modern shogi, the term "Aigakari" typically refers to the closed bishop diagonal variations. One reason is that the open bishop diagonal position shown in the adjacent diagram most often develops into a Side Pawn Capture opening (where the pawn on 34 or 76 are captured by a rook) instead an open bishop diagonal variation of Double Wing. This a major change from past centuries where Side Pawn Capture openings were uncommon as they were considered to be inferior strategies. In the second half of the 20th century, Side Pawn Capture was recognized as a sound opening – eventually becoming one of the most popular Double Static Rook opening among professional players in the 21st century. Thus, a presentday Double Wing opening typically arises when the rook pawns are traded off the board first and only after this are the bishop diagonals opened (if they are opened at all), which prevents the Side Pawn Capture opening possibility. Consequently, the open bishop diagonal variations of Double Wing (without captured side pawns) are not commonly played by professional players (although they are occasionally seen).
Closed bishop diagonal variation.
The initial aim of the Double Wing Attack to threaten each opponent's bishop and to exchange the rook pawns activating the rook.
From the start of the game, the pawn at the head of each bishop (23 for White, 87 for Black) is undefended. By advancing each player's rook pawn (at 83 for White, at 27 for Black), each player aims for their opponent's bishop head. Then, the initial strategy involved is to provide the head of the bishop with adequate defense and/or give the bishop an escape route from the advancing pawn.
1. P-26 P-84, 2. P-25 P-85. Following the joseki, both sides advance their rook pawns to the middle rank 5.
3. G-78 G-32. Then, golds are moved to defend each bishop's head.
Note that the head of the bishop should be defended. (See: § Protecting the bishops below.) 
An alternative option is to advance the side or edge pawn allowing the bishop to escape, which leads to a different set of josekis. 
Another alternative is for Black to play a Floating Rook strategy and prevent White from making a pawn exchange, although Black's Rook becomes somewhat exposed.
4. P-24 Px24, 5. Rx24. Next, Black initiates a pawn attack on the second file leading to a pawn exchange involving the rook.
5. ...P*23. White responds by dropping a pawn to 23 to push Black's rook back.
After move 10 of Double Wing Attack, the joseki branches off into different variants depending on which square the rook moves down to. (See § Further strategy branches below.)
Protecting the bishops.
After advancing the rook pawns in the first four moves (1. P-26 P-84, 2. P-25 P-85), it is important for both players to protect the bishop's head with a gold rather than immediately pushing their rook pawns again. If either player ignores protecting the bishop after move 4 (a blunder), their bishop may get trapped by a pawn drop according to the famous josekis explained below.
Blunder: Black's failure to defend.
5.P-24. This sequence of moves (1.P-26 P-84 2.P-25 P-85 3.P-24) is known as the 5-Move Bomb (五手爆弾 "go-te bakudan").
If Black continues advancing their rook pawn for an attack at 23 instead of defending with a gold as recommended by joseki, after a pawn exchange on the second file involving the rook, White can start a counterattack by advancing their pawn on the eighth file.
Once Black captures White's pawn at 86, rather than recapturing Black's pawn with their rook (as happens in the recommended joseki), White can instead drop a pawn at the bishop's head (P*87). Black's bishop is now trapped.
Although Black can also attack White's bishop with a pawn drop, White now has the initiative and will capture Black's bishop first. 
After Black captures White's "tokin" at 88 with a silver, with a bishop in hand, White can fork Black's rook and king pawn by dropping their bishop to 35. At this point, the choices for Black's rook are to either retreat or attack White's bishop. (Ignoring the bishop attack may lead to losing the rook.) If Black's rook retreats to rank 8 (as shown in the diagram), then White can promote their bishop to a horse at 57.
From here, Black can finally capture White's bishop. 
Rook capturing tokin variant.
After move 17, White must remove the threat of Black's "tokin" on 22.
Aono and Kitao recommend taking the "tokin" with the rook, which is defended by a silver.
From here, Black may exchange rooks.
But, White is in a better position.
Silver capturing tokin variant.
Kitao suggests that taking the "tokin" with White's silver after move 17 is an inferior move. The reason is because Black will drop their bishop to 45 threatening to promote at 63 and further attack White's camp.
At this point, White needs to defend 63. However, a gold can't be used for this purpose because that gold would come between the rook and the silver on rank 2 allowing Black to take the silver for free. Therefore, the suggested move is to use the rook to defend both 63 and 22.
Black now drops a pawn to put the king in check. This move tries to force White's rook to capture the pawn at 52 with White's rook (Rx52), which will allow Black to successfully promote their bishop at 63 and attack White's rook (Bx63).
Kitao considers this position uncertain compared to the position in which the rook captures the "tokin."
If White uses a gold to capture the checking pawn (Gx52), then the rook's defense of the silver will be blocked allowing Black to capture the silver (Rx22).
However, Hosking has a different suggestion which Kitao does not consider.
Hosking's silver variant.
In contrast to Aono and Kitao, Hosking recommends the silver taking the "tokin" along with the defense of this silver as shown below.
If White captures the "tokin" with the silver, Black can now use their newly obtained bishop in the next move.
Black can drop their bishop to 45 (or 36) attacking 63.
If White tries to defend 63 with another piece now, the rook's defense of the silver on 22 will be blocked allowing Black to capture the silver on 22. Thus, Hosking's solution to defend the silver with a gold.
Black's bishop can promote, but White can attack the bishop with a gold now (since the rook is freed up from its defensive role).
Later White may capture more of Black's pawns with either their rook or horse leading to a stronger position for White.
Ignoring White's counterattack.
If Black entertains the possibility of ignoring White completely and continuing their attack by dropping a pawn at 23 to attack White's bishop after move 6, the end result stills favors White.
Both White and Black's pawn can advance and promote, and Black captures the bishop first. White recaptures the "tokin" with a silver. Since White's silver at 22 is defended by their rook and Black has no more pawns to drop, Black's best move is to defend against White's attack by retreating to rank 8. Otherwise, White will capture both Black's bishop as well as Black's silver and promote their rook at the same time.
With Black's rook defending 88, the number of pieces in an exchange is even: White's "tokin" and rook vs Black's silver and rook. However, White can now drop a dangling pawn on 86 which threatens to promote on 87 attacking Black's silver.
At this point, White will capture Black's bishop and break through their camp in future moves leaving Black in an inferior position. Black cannot save their bishop and will probably lose either their knight or silver when White's dangling pawn promotes. There is also the additional risk of White's rook promoting on the eighth file. Although Black was able to get a bishop, White has Black's bishop and also has more captured pieces.
Blunder: White's failure to defend.
According to joseki, when both Black and White defend their bishops with a gold, after White exchanges their rook pawn, Black is recommended to drop a pawn to 8g for move 15 (see above).
However, if Black defends their bishop with gold but White does not and instead continues with a premature advance of their rook pawn, joseki recommends a different strategy.
After White attacks 87 and the subsequent pawn exchange, joseki recommends that Black start a counterattack at the head of White's bishop (23) by the rook pawn rather than trying to defensively attack White's rook. (The reason for this is that White's rook could simply fall back to a floating rook position on 84 thus preventing Black from making a pawn exchange on the second file and then controlling the flow of the game.)
After Black's counterattack, White will capture Black's pawn.
This pawn capture by White allows Black to drop a pawn (obtained from the eighth file pawn exchange) at the head of White's bishop.
White can follow suit and also drop a pawn at the head of Black's bishop.
But, Black captures White's bishop first. 
After White recaptures Black's "tokin" on the second file, Black drops the bishop on 75 creating a rook–pawn fork.
White now has a choice between retreating their rook back to their camp at 82 or attacking Black's bishop.
If the bishop on 75 is attacked (as shown here, R-85), then Black will promote their bishop on fifth file.
White's rook can continue to attack the bishop (R-55), but Black can continue eluding capture with a sidestep to the fourth file. 
White's rook can now promote (R-57+), but Black can defend their king with a pawn drop (P*58).
White can capture another pawn (+Rx47), but Black can gain a knight (+Bx21).
This position is thought to still favor Black.
Blunder: Inferiority of silver.
Using a silver to defend the head of the bishop is considered a "bad formation" (悪形 "akukei") even though it appears somewhat similar in that the silver can defend the head of the bishop. However, a silver cannot defend from the sides leaving the bishop without any direct connection to a defending piece. For instance, in the adjacent diagram, Black moves the left silver beside the bishop to prepare for White's attack from their advancing rook pawn. Due to Black's poor bishop–silver formation, White will now open their bishop's diagonal to take advantage of this situation.
This move by White also prevents Black from opening their diagonal since doing so would lead to White capturing Black's bishop without penalty. Furthermore, this position cannot be easily repaired since Black's silver cannot retreat to its original position.
Further strategy branches.
After move 10 of Double Wing Attack, the joseki branches off into different variants depending on which square the rook moves down to.
In the Floating Rook variant, the rook moves to 26.
In the Retreating Rook variant, the rook returns to 28.
Retreating Rook.
Retreating Rook (引き飛車 "hikihisha") is an opening following the Double Wing opening proper in which Black's rook retreats all the way back to their camp allowing White to also exchange rook pawns.
Floating Rook.
In contrast to the Retreating Rook variant of Double Wing Attack, Floating Rook (浮き飛車) aims to protect rank 6 and prevent White from exchanging pawns on the eighth file to get a pawn in hand.
The translated term "floating" is synonymous with the term "hanging" as in "hanging piece" used in western chess where "floating" means a piece that is not defended by another piece.
Old Double Wing opening.
The Old Double Wing (旧型相掛かり or 旧型相懸り) is rarely seen presently. However, it was the Double Wing variation used up until a little after World War II and early versions of it were used starting from 1625. After the initial opening sequence, unlike modern Double Wing, it is characterized by both players pushing the central pawns (P-56 and ...P-54) supported by a right (attacking) silver behind the central pawn (that is, S-57 and S-53). The Old Double Wing opening became obsolete after central pawn push strategies were replaced by pushing pawns on the fourth file with the Reclining Silver strategy (sixth file for White). 
A joseki for the Old Double Wing opening is the Silver Three Pawn joseki (銀歩三定跡).
Open bishop diagonal variation.
1. P-77. Black opens their bishop diagonal. This is the most flexible first move. 
1. ...P-34. White responds by opening their bishop diagonal as well. 
This variation of Double Wing with both players' bishop diagonals open from the start is a much less common way to play Double Wing. More often than not, this positioning accompanied by dual double rook pawn pushes will lead to a Side Pawn Capture opening or a Tempo Loss Bishop Exchange opening. However, hundreds of years ago, this positioning with all major pieces activated was very typical of the Double Wing Attack. Furthermore, accepting the side pawn gambit was considered a poor move (thus, Side Pawn Capture openings were uncommon) and the modern Tempo Loss Bishop Exchange had not been invented yet.
2. P-24 P-84. Both players give an early indication of choosing a Static Rook opening by advancing their rook pawns. Thus, the game will likely be a Double Static Rook opening.
3. P-25 P-85. Both continue their rook pawn advances to the middle rank 5. Thus, the likelihood of a Double Static Rook game increases. 
4. G-78. Black moves their left gold to the seventh file protecting the 88 square from a later possible bishop attack and protecting the 87 square in preparation for White pawn exchange on that file.
4. ...G-32. White mirrors Black's move for the same reasons. 
Both players have symmetrical positions. 