The chess move known as "en passant" (which is a French phrase meaning "in passing") is very specialized and rarely comes up, but it is a legal and valid move, and is a part of chess just as much as castling is.
First, let us explore a brief history of chess relevant to why the en passant capture even exists, which will help in remembering how it works.
Back in the bad old days, chess was not played by modern rules, and was a relatively dull game. Prior to the modern rule changes (which went into effect during the Renaissance period in Europe), the Queen was one of the weakest pieces on the board, as it could only move one square at a time, much like the king. Pawns only moved one square at a time, even on the first move. There was no castling - this had to be done manually. Also, there was no pawn promotion - a pawn reaching the end of the board simply stopped there. This is not a complete list of differences, but it gives an idea as to what chess was like prior to the modern rules.
Obviously, the lack of a powerful Queen makes the game less exciting, reducing the possibilities for attack. The lack of pawn promotion, while making games less exciting because this threat did not exist, also resulted in many more drawn games, since there would be no way to checkmate with a king and a lone pawn, or to force it at all with any number of pawns. Also, having to take two turns to move any pawn to the fourth row, plus having to take a minimum of four moves to castle by hand, greatly slowed down the early phases of the game.
The game obviously needed some livening-up. The pieces were given more range, the Queen getting the biggest boost, pawns were allowed to promote, and to get rid of the dead weight at the beginning, castling was made a one-move affair, and pawns could advance two squares on the first move. This meant that contact with the enemy pieces would come much sooner in the game. However, one "problem" arose.
Consider the situation where pawns can only move one square forward. This would mean that assuming no pawns had been captured yet, any pawn proceeding forward _must_ eventually risk being captured by an enemy pawn. When the rule change involving pawns being able to move forward two squares took place, this was no longer true. Consider this position:
Black
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8 | | | | | | | | |
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7 | | | | | | | | |
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6 | | | | | | | | |
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5 | | | | | | | | |
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4 | | | | BP | | | | |
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3 | | | | | X | | | |
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2 | | | | | WP | | | |
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1 | | | | | | | | |
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a b c d e f g h
White
where BP = Black Pawn, WP = White Pawn. The letters and numbers along the sides of the board are the coordinates for each square, the lower left being a1, the upper right being h8. The White Pawn is on e2, the Black Pawn on d4.
Notice that the Black Pawn is attacking the square e3, marked with an "X" in the diagram above. No piece can move there without the possibility of being captured by the Black Pawn. Under the old rules, this would restrict the movement of the White Pawn, as it would have to move from e2 to e3 before it could go any farther. However, under the modern rules, since this White Pawn is on its home square, it has the choice of moving two squares forward, to e4, passing right over the e3 square. This would mean that the pawn on d4 would no longer be able to prevent the pawn on e2 from moving forward. This was a side effect the rule changes were not intended to create, so a solution was invented. That solution is en passant.
When a situation like the above occurs (or the reverse situation for Black, with the Black Pawn at home on e7 and the White Pawn d5, for example), and the Pawn on its home square takes the option of moving two squares forward, passing over the square guarded by the enemy pawn, only at this time can an en passant capture be made. In the above example, after the White pawn moves forward two squares, we reach this position:
Black
a b c d e f g h
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8 | | | | | | | | | 8
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7 | | | | | | | | | 7
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6 | | | | | | | | | 6
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5 | | | | | | | | | 5
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4 | | | | BP | WP | | | | 4
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3 | | | | | X | | | | 3
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2 | | | | | | | | | 2
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1 | | | | | | | | | 1
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a b c d e f g h
White
In this example, it is now Black's turn. To rectify the problem that has arisen from the rule changes, Black now has the option of capturing White's Pawn en passant. This is true only for the turn _immediately_ following the White Pawn moving two squares forward, otherwise the option to capture en passant is forfeited.
To execute an en passant capture of White's Pawn on the very next turn, Black pretends (he does not physically move White's Pawn) that the White Pawn only moved one square, not two, and captures the Pawn just as if it were actually standing on the square marked with the "X", e3 in this case. The Black Pawn passes behind the White Pawn, to the square marked by the "X", and the White Pawn is removed from the board. The final position would leave the Black Pawn standing alone on e3, like so:
Black
a b c d e f g h
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8 | | | | | | | | | 8
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7 | | | | | | | | | 7
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6 | | | | | | | | | 6
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5 | | | | | | | | | 5
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4 | | | | | | | | | 4
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3 | | | | | BP | | | | 3
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2 | | | | | | | | | 2
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1 | | | | | | | | | 1
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a b c d e f g h
White
As mentioned above, this type of situation does not come up that often, but when it does, knowing about having this option can mean the difference between winning and losing. For example, consider this position, which you may want to set up on a board in order to follow along:
Black
a b c d e f g h
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8 | | | | | | | BK | | 8
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7 | | | | | | | BP | | 7
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6 | | | | | | | | BP | 6
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5 | | | | | | WP | | | 5
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4 | | | | | | WQ | | WK | 4
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3 | | | | | | | | | 3
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2 | | | BP | | | | | | 2
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1 | | | | | | | | | 1
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a b c d e f g h
White
W = White B = Black P = Pawn Q = Queen K = King
White is obviously winning here, but Black still has one trick left up his sleeve. He can move his pawn from g7 to g5, giving us the following position:
Black
a b c d e f g h
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8 | | | | | | | | BK | 8
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7 | | | | | | | | | 7
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6 | | | | | | | | BP | 6
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5 | | | | | | WP | BP | | 5
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4 | | | | | | WQ | | WK | 4
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3 | | | | | | | | | 3
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2 | | | BP | | | | | | 2
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1 | | | | | | | | | 1
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a b c d e f g h
White
This creates a situation known as a fork, when two pieces are attacked simultaneously by a single piece. The White King is in check, and the White Queen is being threatened at the same time. Also, notice that the Pawn on g5 is protected by the Pawn on h6, which means White cannot simply get out of this situation by capturing the Pawn with his King or Queen. Still, the best that White could do, if he did not know about en passant, would be to capture the Pawn on g5 with his Queen (F4-G5). Black would then capture the Queen with his Pawn (H6-G5), giving check a second time. White would then capture the Pawn with his King (H4-G5). It would then be Black's turn, and White would have no way of stopping Black from queening his Pawn on C2 (C2-C1). This would be enough for Black to win easily.
However, if White knows about en passant, the outcome is entirely different. Going back to the above diagram (after Black has just moved his Pawn forward two squares, forking White's King and Queen), White instead uses his Pawn on F5 to capture Black's Pawn en passant (F5-G6). This saves White's Queen, which is guarding the C1 square, and stops Black from successfully queening his Pawn, since now C2-C1 is now answered by F4-C1. Now it is White who has the easy win.
Please make this file freely available. If you find this file useful, please let me know by sending me (Joe Brooks) mail either through the FidoNet CHESS echo, or at one of the following addresses:
FIDONet net-mail address - 1:2609/202
Internet e-mail address - joe.brooks@newhor.uu.holonet.net.
Other freely available chess tutorial files by me to look for are:
GUIDELNS.TXT - Guidelines to playing better chess for beginners & amateurs.
NOTATION.TXT - A tutorial on the three common forms of chess notation - algebraic, coordinate, and descriptive.
RATINGS.TXT - An explanation of chess ratings & how to get one.
