Greg Strong wrote on Mon, Nov 11, 2019 02:43 AM UTC:
This is tricky, and different players have used different rules. This page states:
According to Pritchard's Popular Chess Variants, 'The en passant rule has seen change. Modern players allow it only when the Pawn advance formed the second move of a turn.'. This helps to eliminate some ambiguity discussed in the comments. (What if a player advanced a Pawn by two squares, then occupied the intermediate square with a piece?)
The situation is potentially even worse than this example. What if a pawn made a double move and then went on to capture a piece with its second move? Would capturing it en passant then magically bring back the piece it captured?
There was a discussion about this on the talk chess site a few years ago. The discussion went on for quite a while and a lot of people weighed in. I can try to dig up the thread, but the final outcome was that there is really only one interpretation of en passant makes sense and doesn't lead to problems: the en passant capture must be made with a player's first move and can only be used to capture a two-space move by the opponent's second move. If a player makes a two-space pawn move with the first of his two moves, it is not subject to en passant. ChessV uses this interpretation and I believe Game Courier should do the same.
This is tricky, and different players have used different rules. This page states:
The situation is potentially even worse than this example. What if a pawn made a double move and then went on to capture a piece with its second move? Would capturing it en passant then magically bring back the piece it captured?
There was a discussion about this on the talk chess site a few years ago. The discussion went on for quite a while and a lot of people weighed in. I can try to dig up the thread, but the final outcome was that there is really only one interpretation of en passant makes sense and doesn't lead to problems: the en passant capture must be made with a player's first move and can only be used to capture a two-space move by the opponent's second move. If a player makes a two-space pawn move with the first of his two moves, it is not subject to en passant. ChessV uses this interpretation and I believe Game Courier should do the same.