In shogi, Bishop Exchange () is a Double Static Rook opening in which the players exchange their bishops relatively early so as to have bishops in hand. Throughout the game, both players have a bishop drop threat, with which they can exploit any positional weakness that their opponent inadvertently creates.
Overview
The Bishop Exchange opening is a Double Static Rook opening. Black starts with activating both their bishop (P-76) and rook (P-26) while White quickly puts pressure on Black with rook pawn pushes (...P-84, ...P-85). White's aim is to exchange their rook pawn off the board on the eighth file as soon as possible. This has a number of benefits: it gives White a pawn in hand that can be used to drop later in the game, and it frees up the rook so that it can move to any rank above Black's camp (see: Sabaki). However, in this opening, in response, Black wishes to prevent White's early rook pawn exchange by defending the 86 square with the bishop via B-77 (and, later after the bishop exchange with the left silver via S-77).
Generally speaking, there is usually this tension of whether to allow rook pawn exchanges and/or bishop exchanges in the major Double Static Rook openings. In contrast to Double Wing Attack and Side Pawn Capture openings, but like the Double Fortress opening, the Bishop Exchange opening prevents each player from trading off each other's rook pawn. Both players move their left silver to the 77 and 33 squares (Black and White, respectively) in order to accomplish this goal in both Bishop Exchange and Fortress openings. (And, Fortress contrasts with Bishop Exchange by preventing an early bishop exchange as well.)
Because the bishops are off the board, each player is restricted in their development compared to other openings since they must not allow any holes where their opponent can drop a bishop.
Certain positions of Bishop Exchange – particularly, the Reclining Silver variations with identical forms for each player – have had a tendency to be drawish in which White wishes for sennichite and Black is unwilling to break the balance of the position. The Bishop Exchange (角換わり, kakugawari) has been one of the most deeply analyzed double static rook openings in professional shogi due to the long-term strategic impact of having bishops in hand.
Castles
Bishop Exchange openings typically use a variant of the Fortress castle known as the Helmet Fortress castle (or just Helmet castle). This castle has the characteristic Fortress form with a silver on 77 and gold on 78. However, unlike the standard Fortress, the Helmet castle keeps the right gold on the 58 square and the sixth file pawn is not necessarily pushed to 66 to make way for the right gold to move to 67. One reason for this difference is that the middlegame fight often starts before this development could be finished. In some situations, when the right offensive knight has jumped to 37 square (or 73 square for White), the gold may need to move from 58 to the 67 square in order to protect the head of the knight (the 36 square) from the opponent's pawn drops.
Another difference is the position of the king initially on the 68 square. Since the bishop is off the board, the king is not restricted in rightward movement to the 69 square like the usual development in the Fortress opening. After the king moves to 68, it may be possible to move the king into the castle to the 88 square via 79. However, again, it often happens that the middlegame will start before the king can make it to 88. Because of this, the king often remains on 68 or on the intermediate 79 square. In some variations, moving to 88 is even not recommended.
The ninth file edge pawn is typically pushed (P-96/...P-14) in this castle since both bishops are in hand and could otherwise be dropped on the ninth or first files. (The standard Fortress often does not have the edge pawn pushed.) However, faster attacking strategies (Climbing Silver, Rushing Silver) may start the offense before the edge pawns are pushed.
Another variation of the castle formation is to have the right gold on the 48 square (62 square for White) where it can defend more area on the right side of the board from the opponent's bishop drop. In the case of a Right King formation in which the king moves rightward away from the Helmet castle, the gold on 48 will be closer to the king, which can rest on the 38 square. This G48/G62 is a form that Lifetime Meijin Yoshio Kimura (now deceased) had experimented with in the first half of the last century and is now experiencing a revival in popularity becoming the most common form found in current (2016–2019) professional shogi.
As for White's Hemlet development, White will often wait for Black to push their rook pawn to the 25 square before moving their castle silver to the 33 square (so that it defends the 24 square preventing a second file pawn exchange). Depending on the game, this may occur fairly late in the opening. Before Black's pawn push, White's silver will usually remain on the 42 square until that time.
Of course, this Helmet castle may be developed into a regular (Gold) Fortress castle if the opponent aims for an attack on the sixth file (or fourth file for White). White may often choose this castle development if they are playing defensively.
In Double Reclining Silver games, White's castle has often been a variant of the Dented Fortress castle (へこみ矢倉 hekomi yagura) with the king directly under the left gold (on 31) and the right gold adjacent to the left gold (on 42) with the pawn above it pushed to allow for development into a Gold Fortress.
More recently in 2018, the emphasis on White's king safety has been relaxed, and the king is placed the 52 square or the 42 square instead of moving to 31 or 22. (Note that if the king is on 42, then this implies that White will position their silver on 22 if Black has not already pushed their rook pawn to 25, which differs from the book lines that use the silver on 42.) The adjacent diagram shows an example of such a position from 2019 – a Double Reclining Silver, P25 R29 G48 Identical Formation variation. At the time of this example, there were over 40 games with this same position in the Japan Shogi Association game record database. White (Toyoshima) moved their king from 42 to 52 opting for losing tempo (since he could have played K-52 from the start position) and waiting for a counterattack over deviating from his ideal configuration, in which White is broadly defending against bishop drops.
Attacking formations
There are a number of attacking strategies used with Bishop Exchange. The three main strategies are classified by the configuration of the right offensive silver: Reclining Silver, Climbing Silver, Rushing Silver.
The most popular strategy among professional players is the Reclining Silver variation where the right silver is advanced to the 56 square (54 square for White) via 47. Typically, the right knight is also advanced to the 37 square for the attack. The rook pawn may be advanced to the 25 square although it is also possible that it is left on the 26 square so that the knight can jump to 25. In some cases, moving the silver to 56 very early can be problematic.
The traditional form of Reclining Silver uses a Helmet castle with the right gold on the 58 square. This position allows a possible attack on the fourth file by having the rook shift to the 48 square. A more recent Reclining Silver position is the P-25 · G-48 · R-29 variation. This positions the right gold on 48 instead of 58 and the rook moves down to the 29 square (if left on 28 then the gold and rook are susceptible to an opponent's fork by dropping their bishop to the 39 square).
Due to its involvement of several pieces, Reclining Silver is the most deliberate out of the three silver strategies and it is balanced with both offensive and defensive characteristics. In comparison, the other two silver strategies have a stronger attacking character. Several entire books have been written on solely Reclining Silver Bishop Exchange variations.
The Climbing Silver variation places the silver on the second file on 26 (or 84 for White) usually aiming for an attack on the first file followed by the second file but also sometimes aiming for an attack on the third file instead. Castle development in this strategy can often be very minimal even with leaving the king in its start position before attacking. This the quickest attack out of the three silver strategies. The player may also start their attack before pushing edge pawns.
The Rushing Silver variation places the silver on the 46 square (or White's 64 square) after moving through the 37 square, which has been cleared with a P-36 pawn push. The aim is to attack starting on the third file moving diagonally to the second file. An issue with the P-36 configuration is that once the offensive silver has moved for its attack, the rook on 28 becomes susceptible to the opponent's bishop attacks through the 37 square.
A fourth and less common strategy is the defensive Right King. This strategy flouts the general shogi maxim that the king and rook should be far apart by having the king next to the rook. Here, the rook is used as a defense piece protecting the bottom rank. The left silver may be used for offense. Right King is more commonly played by White but uncommon for Black in the regular Bishop Exchange opening. However, in the Tempo Loss Bishop Exchange opening, it is more frequently played by Black than in regular Bishop Exchange and thought to be effective there.
Either player may choose any combination of these strategies. However, there are recommended strategies for each player in some positions.
Admonishment against fifth file pawn pushes
In the shogi of the early part of the twentieth century, fifth file pawn pushes – P-56 and ...P-54 – were very common. They were found in Fortress openings as well as many Double Wing Attack (see: §Old Double Wing) and Snowroof games. However, eventually, shogi theory found them problematic in Bishop Exchange openings due to the forking capabilities of the bishops in hand. (They also dropped out of favor in Double Wing Attack openings as well.)
History
The Bishop Exchange, Reclining Silver variation was a popular opening up until the 1950s. However, it became prone to draws due sennichite and lost its popularity. After this, Climbing Silver and Rushing Silver variations became popular in the 1960s and 1970s.
Main book line: White's bishop trade with ☖...P-85
The main book line for Bishop Exchange starts with 1.P-76 P-84 2.P-26 P-85 3.B-77 P-34 4.S-88 G-32 5.G-78 Bx77+ 6.Sx77 S-42 7.S-38.
1.P-76. Black chooses the flexible option of activating their bishop. However, Black can start with 1.P-26 as well.
1...P-84. White pushes their rook pawn showing their intent to play Static Rook and gives both players the possibility of a Bishop Exchange opening.