Answers.Body: <p>A pawn which 'exposes' itself to en passant will be distinct from one that doesn't. Say, for a black pawn on a4 and a white pawn on b4: giving black the option of playing ...axb3 at any time after b2-b4 means that the moves b2-b4 and b3-b4, which seemingly lead to the same position, have long-term implications as it depends too much on the past. In particular, when the pawn stands on b2, white would need to weigh the pros and cons of moving his pawn to b4 in one or two moves (where the con is giving black a long-lasting right to take on b3).</p>
  
  <p>Or, in the spirit of the motivation of en passant that you correctly described, permanently stopping a two-square pawn from 'escaping' would greatly neglect (or complicate) the benefit of being able to move two squares forward.</p>
  
  <p>Another practical reason because it's just much simpler for humans: with more than one pawn on the board, it would be complicated to keep track of en passant rights on all of them. (Someone once suggested that pieces be able to capture en passant at some FIDE meeting; needless to say, it was laughed out of the room --- that said, I can't recall the source of this story.)</p>
  
  <p>One may ask a similar (opposite?) question about castling rights; the 'simpler' option may arguably be to always allow castling as long as the pieces are back on their original squares, but that too has strong implications, one of which is a reduced importance of development.</p>
  
  <p>Generally, the special moves in chess have restrictions associated with them to ensure that they don't change the way the game 'looks' after the time these special moves are made (playing ...axb3 to take a white pawn on b4 looks completely unlike the old game, so it's only allowed for one move). After all, the rules were designed by humans, arguably arbitrarily.</p>
  
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Answers.UserDisplayName: Ken Wei
Answers.Body: <p>The reason is very simple. If white has a pawn on d5 preventing a black pawn on e7 from moving then the move e7-e5 circumvents that. To keep things fair when the double pawn first move was introduced then the en passant rule was also introduced so that white wouldn't lose the possibility to capture. However going back to the pre-double move era if black plays e7-e6 and white doesn't capture immediately then black can play e6-e5 on the next move and white has lost his chance forever. That is reflected in the restriction of en passant to the move immediately following the double pawn move.</p>
  
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Answers.Text: Is this supported by sources?
Answers.UserDisplayName: user1583209
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Answers.Text: When you say "double pawn move", do you mean simply moves 2 squares? or **takes two moves instantly?** If the latter, that would explain things, but I've always thought of the pawn as having a range of 2 instead of taking two moves in one. If you can find a source, please post it.
Answers.UserDisplayName: DrZ214
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Answers.UserDisplayName: Brian Towers
Answers.Body: <p>Interesting question, though I am not sure there is a good answer which could be backed by sources. I can think of the following motivations:</p>
  
  <ol>
  <li>If you imagine your pawns to be real "armies", then if one army is passing you at high speed (double step), you need to act immediately (if you wish to do so) and cannot wait before the other army is far away.</li>
  <li>Practicality. Keeping track of all possible en passant captures could become tricky. What if one or both of the pawns involved get replaced by another pawn of the same color...</li>
  <li>In chess, if a piece attacks a piece of the same type, the other piece at the same time attacks the first piece. With indefinite en passant you would create an asymmetry where one pawn could capture another pawn but not vice versa.</li>
  <li>Indefinite en passant would be too big a change to the game as it was known before the introduction of en passant.</li>
  <li>(see Ken Wei's answer): With a black pawn on a4 and a white pawn on b2. You need to compare the option of playing b2-b3-b4 in two moves (which would lose you a tempo) with the double step (which would put you in danger of indefinite capture). </li>
  </ol>
  
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Answers.Text: Thank you, I did not understand Ken Wei's first paragraph until you explained it. Indefinite danger of capture. This whole ep business adds a lot more tricky stuff than meets the eye.
Answers.UserDisplayName: DrZ214
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Answers.UserDisplayName: user1583209
Answers.Body: <p>Regarding the en passant rule requiring the capture being made immediately or not
  at all is due to a situation where if an opposing piece on the next move moved onto the vacant square where the en passant would occur, then TWO captures could be made
  at ONCE with one being done by en passant and the other being a normal pawn capture.</p>
  
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Answers.UserDisplayName: Bob Woodworth
Answers.Body: <p>Your programming description relies on variable name specific to your program, so it's hard to understand what you're saying, but one way to program this is to to have a "double pawn move" boolean. To makes things simpler, let's assume that white is the one making a double pawn move. To move a pawn two space, white first moves the pawn <em>one</em> space, and set that pawn's "double move" boolean to true. If any of the opponent's pawns have a true "double pawn move" booleans, then they have only two options: take or pass. So now black has only two legal move (three if two pawns are adjacent). Black can either capture the pawn, or pass. If black passes, then it's now white's turn. But if a player starts their turn with a "double pawn move" boolean being true for any of their own pawns, they aren't allowed to make any move other than advancing that pawn, and then the boolean is set to false. If black captures the pawn, then the boolean is set to false, and it's now white's turn. </p>
  
  <p>In other words, you can think of <em>en passant</em> not being that white makes one move consisting of the pawn moving two spaces, and then black has the <em>en passant</em> option only on the next move, but rather that white makes a move consisting of the pawn moving one space, black makes a move consisting of either capturing that pawn or passing, and then, if the pawn is still on the board, white makes a move of advancing the pawn again. The <em>en passant</em> option exists only for the intermediate move (the one in which black either captures or passes) because that's the only move where the pawn is in the right location to be captured.</p>
  
  <p>However, there is a corner case presented here: <a href="https://www.futilitycloset.com/2010/06/09/intercepted/" rel="nofollow noreferrer">https://www.futilitycloset.com/2010/06/09/intercepted/</a> . As <em>en passant</em> is currently implemented, it's legal to make a double move if you kings is not in check after the second advancement, even if you king is in check after the pawn is advanced only one space. So your code would have to take that into account.</p>
  
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Answers.Text: If before moving the pawn one space, White announces the intention to move it twice consecutively, all of Black's pieces except a possible capturing pawn would be immobilized until Black has either captured the pawn or passed, and in the former case White would be allowed to make any move (in the latter case, White would have to move the pawn again and put Black on move).  Thus, White couldn't be in check after the first pawn move.  A more interesting aspect of the rule would be that an announced double pawn move would be illegal if a mandated follow-up would leave a player in check.
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Answers.UserDisplayName: Acccumulation
Answers.Body: <p>The OP asked me to provide sources supporting the claim that the double step was originally a <em>double move</em>. I'll do my best.</p>
  
  <p>The immediate predecessor of modern chess, the game <a href="https://en.wikipedia.org/wiki/Shatranj" rel="noreferrer">Shatranj</a>, did have pawns (<em>sarbaz</em>), but no double step. The pieces in Shartanj had relatively weak powers, and gameplay, especially in the opening, was generally slow.</p>
  
  <p>In medieval times they addressed this problem by starting the game from certain <a href="https://en.wikipedia.org/wiki/Shatranj#Openings" rel="noreferrer">tabiyas</a>, battle arrays where the opposing forces were already in contact.</p>
  
  <p>A more recent western invention to speed up the play was to allow double-initial moves of pawns.
  From <a href="https://books.google.no/books?isbn=0307828298" rel="noreferrer">A Short History of Chess</a> by Henry A. Davidson, Chapter 7:</p>
  
  <blockquote>
    <p><em>The Initial Double Move</em>. Not until about 1550 did the pawn uniformly enjoy the power of moving two squares for his first move. The reason for the final adoption of the rule was simply that openings became conventionalized. White moved P to K3 [<code>e3</code>]; Black did likewise. Then each pushed the pawn up a square. Since this was the usual opening, the modern practice of allowing both moves at once, thus speeding up the game, was introduced. As a matter of fact, this pawn privilege was the last, not the first, of a series of special consecutive-move privileges.</p>
  </blockquote>
  
  <p>Davidson then very briefly discusses consecutive-move rules in earlier versions of chess.</p>
  
  <p>There was also at some point an option to move two different pawns simultaneously one step each. Quote from the same book:</p>
  
  <blockquote>
    <p>During the seventeenth century a convention was in effect permitting a player to open a game by moving both rooks' pawns simultaneously one square each.
    [...]
    Perhaps the idea was that moving two pawns each one square was the equivalent of moving one pawn two squares.</p>
  </blockquote>
  
  <p>I think this historical context provides some rationale for why the en passant rule was introduced. If the double step is actually a double move, then it's quite logical that the opponent should be given a chance (on the next move) to capture the pawn after the first step. (See also Brian Towers' answer.)</p>
  
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2017-05-24T11:43:07.543Z
Answers.Text: What does "one step each with two separate pawns" mean?
Answers.UserDisplayName: DrZ214
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Answers.Text: I hope it makes more sense now. Let me try to find a direct quote from the book.
Answers.UserDisplayName: Dag Oskar Madsen
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Answers.Text: Ah, do you mean 2 pawns could move simultaneously on the same turn, but only 1 step forward each? That would be **extremely** radical in today's context, far more so than the weird *en passant.*
Answers.UserDisplayName: DrZ214
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Answers.Text: Yes, that's what I meant. My whole argument is that in the earlier game Shatranj pawns could only move one step, and the double step we have in modern chess is through the evolution of the game really two Shatranj pawn moves in a row. I'll back that up with quotes when I find them.
Answers.UserDisplayName: Dag Oskar Madsen
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Answers.Text: Also see https://chess.stackexchange.com/questions/4414/why-is-castling-through-and-out-of-check-prohibited?noredirect=1&lq=1
Answers.UserDisplayName: BlindKungFuMaster
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Answers.Text: Just curious: when the simultaneous pawn moves were allowed, was it only for rook pawns? If so, how strange. Those are the worst moves to start with unless you're trying a rook lift, also dumb. BTW thanks for the great answer.
Answers.UserDisplayName: DrZ214
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Answers.Text: @DrZ214 From Davidson's book it seems it was only allowed for rook pawns. It is not easy to guess what source he used since all sources are just listed at the end of the book with no references in the text itself.
Answers.UserDisplayName: Dag Oskar Madsen
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Answers.UserDisplayName: Dag Oskar Madsen
Body: <p>Quoting <a href="http://www.fide.com/fide/handbook?id=124&amp;view=article" rel="nofollow noreferrer">FIDE rulebook</a>:</p>
  
  <blockquote>
    <p>This capture is <strong>only legal on the move following this advance</strong> and is called an ‘en passant’ capture.</p>
  </blockquote>
  
  <p>Quoting <a href="https://en.wikipedia.org/wiki/En_passant" rel="nofollow noreferrer">Wikipedia</a>:</p>
  
  <blockquote>
    <p>The <em>en passant</em> capture must be made <strong>at the very next turn</strong> or the right to do so is lost.</p>
  </blockquote>
  
  <p><strong>Why?</strong></p>
  
  <hr>
  
  <p>I know the basic story of <em>en passant</em> and I'll just paraphrase it here: A rule was added to let pawns move two squares forward on its first move. Someone said something like, what if a pawn uses this to escape from an enemy adjacent pawn? Then the basic pawn skeleton/framework would be changed! So we must have an <em>en passant</em> to preserve the same chess framework.</p>
  
  <p>Okay that part makes sense, but why does it have to be <strong>on the very next move?</strong> Was that part of the rule only added later? Why was it added at all? When two pawns are diagonally adjacent confronting each other, they don't have to capture immediately or else forefeit the ability to capture. So the concept of the "ghost pawn", if you don't capture it immediately, and the other player doesn't move it, seems to me like it should still be there and available for capture as long as it's there.</p>
  
  <p><strong>Does anyone know why this part of <em>en passant</em> was created?</strong></p>
  
  <hr>
  
  <p><strong>Optional Background:</strong> In case you're curious, I come from a programming background. Of all the special moves, <em>en passant</em> is (was, see below) the hardest to program. Not only is it the only move where the capturing piece moves to a blank square, but it's the only move that "disappears" if you don't take it. That means it's a move where <strong>the total board position is not enough information</strong> to generate all moves. (That's true for castling too, which already has its fair share of tricky rules.) Two enemy pawns could be adjacent on the 4th or 5th rank, surrounded by blank squares, but you still don't know if <em>en passant</em> is possible. You have to check the previous move.</p>
  
  <p>Or so I thought. Now that I'm revisiting this, I figured out a clever way to implement <em>en passant</em> without checking the move log. Don't generate it at turn start. Generate it at move_take, specifically at the end of a 2-step pawn move. But store it in a special move array since the normal one gets cleared at every inc_turn. Then migrate and clear appropriately at gen_all_moves. That way, if ep is not taken, it doesn't generate itself on the next move because you didn't move that same pawn two steps on the next move. Just remember to check both sides of the pawn and create <strong>two</strong> ep moves if two enemy pawns are there.</p>
  
  <p>Castling was implemented with the help of a "moved" boolean in every piece, which is also useful for a pawn's first move of course. Even if the King and Rook are in the right spot, you use that to see if the King/Rook has moved before. No need to check the move log. But I will say that Castling is now the hardest move to implement because you gotta check if the path is under attack. En passant is no longer the hardest now that I thought of the clever way.</p>
  
Comments.ContentLicense: CC BY-SA 3.0
Comments.CreationDate: 2017-05-22T07:57:53.620Z
Comments.Text: "*it's the only move where the total board position is not enough information to generate all moves*" That would also apply to castling: If the king or respective rook moved during the game and returned to its original square you are not allowed to castle.
Comments.UserDisplayName: user1583209
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Comments.CreationDate: 2017-05-22T08:45:35.953Z
Comments.Text: @user1583209 Ah yes that's right. I should check all my chess games for that bug.
Comments.UserDisplayName: DrZ214
Comments.ContentLicense: CC BY-SA 3.0
Comments.CreationDate: 2017-05-22T10:03:40.583Z
Comments.Text: Depending on what you are programming, you also need to take care of threefold repetition and 50 move rules which are not apparent from the board position either.
Comments.UserDisplayName: user1583209
Comments.ContentLicense: CC BY-SA 3.0
Comments.CreationDate: 2017-05-22T13:57:05.090Z
Comments.Text: The commonly used Forsyth-Edwards Notation (FEN) for a chess position includes whether or not en passant and/or castling are legal in that position.  The board may not be enough information, but a FEN is.  https://en.wikipedia.org/wiki/Forsyth%E2%80%93Edwards_Notation
Comments.UserDisplayName: Ghotir
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Comments.Text: It's quite clear historically the double step was originally a double *move*. I think this explains the logic behind the en passant rule.
Comments.UserDisplayName: Dag Oskar Madsen
Comments.ContentLicense: CC BY-SA 3.0
Comments.CreationDate: 2017-05-23T09:41:15.703Z
Comments.Text: Note that removing the "on the very next move" rule of en-passant won't save you from having to carry additional information about the position - you still need to know whether the target pawn got to the 5/4th rank via a double move or two single moves. Unless you are proposing that you should be able to "en-passant" on single moves too?
Comments.UserDisplayName: firtydank
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Comments.CreationDate: 2017-05-24T06:32:07.017Z
Comments.Text: @DagOskarMadsen Yes that would explain it, but I've never seen it explicitly called a double move. Can you post a link to a historical source for that? And if so please make it into an answer and I'll accept it since this is the only 100% explanatory explanation that I can see. I'll just add that it's not obvious that it's **thought of** as a double move. B/R/Q can also do "double moves" because of their range. I've always thought of the pawn having a modified range rather than a double move.
Comments.UserDisplayName: DrZ214
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Comments.CreationDate: 2019-08-30T01:47:49.050Z
Comments.Text: Why do bishops move in a diagonal?
Comments.UserDisplayName: Inertial Ignorance
Tags: <history><en-passant>
Title: Why is en passant only possible on the first opportunity?
ThreadText: # Why is en passant only possible on the first opportunity?
  
  (Asked by: username_0 on 2017-05-22)
  
  Quoting [FIDE rulebook](http://www.fide.com/fide/handbook?id=124&view=article):
  
  > This capture is **only legal on the move following this advance** and is called an ‘en passant’ capture.
  
  Quoting [Wikipedia](https://en.wikipedia.org/wiki/En_passant):
  
  > The *en passant* capture must be made **at the very next turn** or the right to do so is lost.
  
  **Why?**
  
  ---
  
  I know the basic story of *en passant* and I'll just paraphrase it here: A rule was added to let pawns move two squares forward on its first move. Someone said something like, what if a pawn uses this to escape from an enemy adjacent pawn? Then the basic pawn skeleton/framework would be changed! So we must have an *en passant* to preserve the same chess framework.
  
  Okay that part makes sense, but why does it have to be **on the very next move?** Was that part of the rule only added later? Why was it added at all? When two pawns are diagonally adjacent confronting each other, they don't have to capture immediately or else forefeit the ability to capture. So the concept of the "ghost pawn", if you don't capture it immediately, and the other player doesn't move it, seems to me like it should still be there and available for capture as long as it's there.
  
  **Does anyone know why this part of *en passant* was created?**
  
  ---
  
  **Optional Background:** In case you're curious, I come from a programming background. Of all the special moves, *en passant* is (was, see below) the hardest to program. Not only is it the only move where the capturing piece moves to a blank square, but it's the only move that "disappears" if you don't take it. That means it's a move where **the total board position is not enough information** to generate all moves. (That's true for castling too, which already has its fair share of tricky rules.) Two enemy pawns could be adjacent on the 4th or 5th rank, surrounded by blank squares, but you still don't know if *en passant* is possible. You have to check the previous move.
  
  Or so I thought. Now that I'm revisiting this, I figured out a clever way to implement *en passant* without checking the move log. Don't generate it at turn start. Generate it at move\_take, specifically at the end of a 2-step pawn move. But store it in a special move array since the normal one gets cleared at every inc\_turn. Then migrate and clear appropriately at gen\_all\_moves. That way, if ep is not taken, it doesn't generate itself on the next move because you didn't move that same pawn two steps on the next move. Just remember to check both sides of the pawn and create **two** ep moves if two enemy pawns are there.
  
  Castling was implemented with the help of a "moved" boolean in every piece, which is also useful for a pawn's first move of course. Even if the King and Rook are in the right spot, you use that to see if the King/Rook has moved before. No need to check the move log. But I will say that Castling is now the hardest move to implement because you gotta check if the path is under attack. En passant is no longer the hardest now that I thought of the clever way.
  
  **Tags:** history, en-passant
  
  > **Comments:**
  > **username_1**: "*it's the only move where the total board position is not enough information to generate all moves*" That would also apply to castling: If the king or respective rook moved during the game and returned to its original square you are not allowed to castle.  
  > **username_0**: @username_1 Ah yes that's right. I should check all my chess games for that bug.  
  > **username_1**: Depending on what you are programming, you also need to take care of threefold repetition and 50 move rules which are not apparent from the board position either.  
  > **username_2**: The commonly used Forsyth-Edwards Notation (FEN) for a chess position includes whether or not en passant and/or castling are legal in that position.  The board may not be enough information, but a FEN is.  https://en.wikipedia.org/wiki/Forsyth%E2%80%93Edwards_Notation  
  > **username_3**: It's quite clear historically the double step was originally a double *move*. I think this explains the logic behind the en passant rule.  
  > **username_4**: Note that removing the "on the very next move" rule of en-passant won't save you from having to carry additional information about the position - you still need to know whether the target pawn got to the 5/4th rank via a double move or two single moves. Unless you are proposing that you should be able to "en-passant" on single moves too?  
  > **username_0**: @DagOskarMadsen Yes that would explain it, but I've never seen it explicitly called a double move. Can you post a link to a historical source for that? And if so please make it into an answer and I'll accept it since this is the only 100% explanatory explanation that I can see. I'll just add that it's not obvious that it's **thought of** as a double move. B/R/Q can also do "double moves" because of their range. I've always thought of the pawn having a modified range rather than a double move.  
  > **username_5**: Why do bishops move in a diagonal?  
  
  ---
  
  ## 6 answers
  
  ### Answer by username_3 on 2017-05-24. Score: 9 | Accepted answer
  
  The OP asked me to provide sources supporting the claim that the double step was originally a *double move*. I'll do my best.
  
  The immediate predecessor of modern chess, the game [Shatranj](https://en.wikipedia.org/wiki/Shatranj), did have pawns (*sarbaz*), but no double step. The pieces in Shartanj had relatively weak powers, and gameplay, especially in the opening, was generally slow.
  
  In medieval times they addressed this problem by starting the game from certain [tabiyas](https://en.wikipedia.org/wiki/Shatranj#Openings), battle arrays where the opposing forces were already in contact.
  
  A more recent western invention to speed up the play was to allow double-initial moves of pawns.
  From [A Short History of Chess](https://books.google.no/books?isbn=0307828298) by Henry A. Davidson, Chapter 7:
  
  > *The Initial Double Move*. Not until about 1550 did the pawn uniformly enjoy the power of moving two squares for his first move. The reason for the final adoption of the rule was simply that openings became conventionalized. White moved P to K3 [`e3`]; Black did likewise. Then each pushed the pawn up a square. Since this was the usual opening, the modern practice of allowing both moves at once, thus speeding up the game, was introduced. As a matter of fact, this pawn privilege was the last, not the first, of a series of special consecutive-move privileges.
  
  Davidson then very briefly discusses consecutive-move rules in earlier versions of chess.
  
  There was also at some point an option to move two different pawns simultaneously one step each. Quote from the same book:
  
  > During the seventeenth century a convention was in effect permitting a player to open a game by moving both rooks' pawns simultaneously one square each.
  > [...]
  > Perhaps the idea was that moving two pawns each one square was the equivalent of moving one pawn two squares.
  
  I think this historical context provides some rationale for why the en passant rule was introduced. If the double step is actually a double move, then it's quite logical that the opponent should be given a chance (on the next move) to capture the pawn after the first step. (See also username_7' answer.)
  
  > **Comments:**
  > **username_0**: What does "one step each with two separate pawns" mean?  
  > **username_3**: I hope it makes more sense now. Let me try to find a direct quote from the book.  
  > **username_0**: Ah, do you mean 2 pawns could move simultaneously on the same turn, but only 1 step forward each? That would be **extremely** radical in today's context, far more so than the weird *en passant.*  
  > **username_3**: Yes, that's what I meant. My whole argument is that in the earlier game Shatranj pawns could only move one step, and the double step we have in modern chess is through the evolution of the game really two Shatranj pawn moves in a row. I'll back that up with quotes when I find them.  
  > **username_11**: Also see https://chess.stackexchange.com/questions/4414/why-is-castling-through-and-out-of-check-prohibited?noredirect=1&lq=1  
  > **username_0**: Just curious: when the simultaneous pawn moves were allowed, was it only for rook pawns? If so, how strange. Those are the worst moves to start with unless you're trying a rook lift, also dumb. BTW thanks for the great answer.  
  > **username_3**: @username_0 From Davidson's book it seems it was only allowed for rook pawns. It is not easy to guess what source he used since all sources are just listed at the end of the book with no references in the text itself.  
  
  ---
  
  ### Answer by username_6 on 2017-05-22. Score: 9
  
  A pawn which 'exposes' itself to en passant will be distinct from one that doesn't. Say, for a black pawn on a4 and a white pawn on b4: giving black the option of playing ...axb3 at any time after b2-b4 means that the moves b2-b4 and b3-b4, which seemingly lead to the same position, have long-term implications as it depends too much on the past. In particular, when the pawn stands on b2, white would need to weigh the pros and cons of moving his pawn to b4 in one or two moves (where the con is giving black a long-lasting right to take on b3).
  
  Or, in the spirit of the motivation of en passant that you correctly described, permanently stopping a two-square pawn from 'escaping' would greatly neglect (or complicate) the benefit of being able to move two squares forward.
  
  Another practical reason because it's just much simpler for humans: with more than one pawn on the board, it would be complicated to keep track of en passant rights on all of them. (Someone once suggested that pieces be able to capture en passant at some FIDE meeting; needless to say, it was laughed out of the room --- that said, I can't recall the source of this story.)
  
  One may ask a similar (opposite?) question about castling rights; the 'simpler' option may arguably be to always allow castling as long as the pieces are back on their original squares, but that too has strong implications, one of which is a reduced importance of development.
  
  Generally, the special moves in chess have restrictions associated with them to ensure that they don't change the way the game 'looks' after the time these special moves are made (playing ...axb3 to take a white pawn on b4 looks completely unlike the old game, so it's only allowed for one move). After all, the rules were designed by humans, arguably arbitrarily.
  
  
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  ### Answer by username_7 on 2017-05-23. Score: 4
  
  The reason is very simple. If white has a pawn on d5 preventing a black pawn on e7 from moving then the move e7-e5 circumvents that. To keep things fair when the double pawn first move was introduced then the en passant rule was also introduced so that white wouldn't lose the possibility to capture. However going back to the pre-double move era if black plays e7-e6 and white doesn't capture immediately then black can play e6-e5 on the next move and white has lost his chance forever. That is reflected in the restriction of en passant to the move immediately following the double pawn move.
  
  > **Comments:**
  > **username_1**: Is this supported by sources?  
  > **username_0**: When you say "double pawn move", do you mean simply moves 2 squares? or **takes two moves instantly?** If the latter, that would explain things, but I've always thought of the pawn as having a range of 2 instead of taking two moves in one. If you can find a source, please post it.  
  
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  ### Answer by username_1 on 2017-05-22. Score: 3
  
  Interesting question, though I am not sure there is a good answer which could be backed by sources. I can think of the following motivations:
  
  1. If you imagine your pawns to be real "armies", then if one army is passing you at high speed (double step), you need to act immediately (if you wish to do so) and cannot wait before the other army is far away.
  2. Practicality. Keeping track of all possible en passant captures could become tricky. What if one or both of the pawns involved get replaced by another pawn of the same color...
  3. In chess, if a piece attacks a piece of the same type, the other piece at the same time attacks the first piece. With indefinite en passant you would create an asymmetry where one pawn could capture another pawn but not vice versa.
  4. Indefinite en passant would be too big a change to the game as it was known before the introduction of en passant.
  5. (see username_6's answer): With a black pawn on a4 and a white pawn on b2. You need to compare the option of playing b2-b3-b4 in two moves (which would lose you a tempo) with the double step (which would put you in danger of indefinite capture).
  
  > **Comments:**
  > **username_0**: Thank you, I did not understand username_6's first paragraph until you explained it. Indefinite danger of capture. This whole ep business adds a lot more tricky stuff than meets the eye.  
  
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  ### Answer by username_8 on 2022-08-08. Score: 2
  
  Regarding the en passant rule requiring the capture being made immediately or not
  at all is due to a situation where if an opposing piece on the next move moved onto the vacant square where the en passant would occur, then TWO captures could be made
  at ONCE with one being done by en passant and the other being a normal pawn capture.
  
  
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  ### Answer by username_9 on 2019-08-29. Score: 1
  
  Your programming description relies on variable name specific to your program, so it's hard to understand what you're saying, but one way to program this is to to have a "double pawn move" boolean. To makes things simpler, let's assume that white is the one making a double pawn move. To move a pawn two space, white first moves the pawn *one* space, and set that pawn's "double move" boolean to true. If any of the opponent's pawns have a true "double pawn move" booleans, then they have only two options: take or pass. So now black has only two legal move (three if two pawns are adjacent). Black can either capture the pawn, or pass. If black passes, then it's now white's turn. But if a player starts their turn with a "double pawn move" boolean being true for any of their own pawns, they aren't allowed to make any move other than advancing that pawn, and then the boolean is set to false. If black captures the pawn, then the boolean is set to false, and it's now white's turn.
  
  In other words, you can think of *en passant* not being that white makes one move consisting of the pawn moving two spaces, and then black has the *en passant* option only on the next move, but rather that white makes a move consisting of the pawn moving one space, black makes a move consisting of either capturing that pawn or passing, and then, if the pawn is still on the board, white makes a move of advancing the pawn again. The *en passant* option exists only for the intermediate move (the one in which black either captures or passes) because that's the only move where the pawn is in the right location to be captured.
  
  However, there is a corner case presented here: <https://www.futilitycloset.com/2010/06/09/intercepted/> . As *en passant* is currently implemented, it's legal to make a double move if you kings is not in check after the second advancement, even if you king is in check after the pawn is advanced only one space. So your code would have to take that into account.
  
  > **Comments:**
  > **username_10**: If before moving the pawn one space, White announces the intention to move it twice consecutively, all of Black's pieces except a possible capturing pawn would be immobilized until Black has either captured the pawn or passed, and in the former case White would be allowed to make any move (in the latter case, White would have to move the pawn again and put Black on move).  Thus, White couldn't be in check after the first pawn move.  A more interesting aspect of the rule would be that an announced double pawn move would be illegal if a mandated follow-up would leave a player in check.  
  
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