🕸Fergus Duniho wrote on Mon, Nov 11, 2019 08:26 PM UTC:
The earliest source I can find is Pritchard's 1997 Encyclopedia, which is the source Hans originally used to write this page. It says "En passant is legal if the opponent moved a pawn two squares on either of his moves but the capture must be made at once. However, if the opponent made two two-square pawn moves, both pawns can be taken e.p. This last rule is credited to Alekhine by F. Palatz in an article on the subject (LEC Sep 1928)." LEC refers to L'Echiquier. Notably, Alekhine is not one of the inventors, and how the game should handle en passant might be something that the original inventors didn't think of. It is also unknown whether the game was invented by Fortis or by de Queylar. It has been attributed to each one, but its origins are murky.
Regarding one of the alternative rules, it says "The game was sometimes played with alternative rules: a check on the first move was illegal and a player could not capture e.p. if the pawn had been moved in the first part of the player's turn." It's very possible that Fortis and de Queylar invented similar games with slightly different rules that eventually got conflated together.
The rule that Greg Strong and H. G. Muller propose has the advantage of being the simplest to program. It works with code that has already been written for Chess. Of course, the original inventor of the game would not have had this in mind, since programming games was not an option when it was invented. However, the rules as initially described above can be programmed, and that's what I have done in Game Courier. The only issue with them is that they need emendation for a Pawn that moves twice on the same turn. If we keep those rules, then en passant capture should be impossible in this instance, or it should be allowed for the Pawn on its new space. I have the former programmed right now, whereas the latter would be trickier to program.
The earliest source I can find is Pritchard's 1997 Encyclopedia, which is the source Hans originally used to write this page. It says "En passant is legal if the opponent moved a pawn two squares on either of his moves but the capture must be made at once. However, if the opponent made two two-square pawn moves, both pawns can be taken e.p. This last rule is credited to Alekhine by F. Palatz in an article on the subject (LEC Sep 1928)." LEC refers to L'Echiquier. Notably, Alekhine is not one of the inventors, and how the game should handle en passant might be something that the original inventors didn't think of. It is also unknown whether the game was invented by Fortis or by de Queylar. It has been attributed to each one, but its origins are murky.
Regarding one of the alternative rules, it says "The game was sometimes played with alternative rules: a check on the first move was illegal and a player could not capture e.p. if the pawn had been moved in the first part of the player's turn." It's very possible that Fortis and de Queylar invented similar games with slightly different rules that eventually got conflated together.
The rule that Greg Strong and H. G. Muller propose has the advantage of being the simplest to program. It works with code that has already been written for Chess. Of course, the original inventor of the game would not have had this in mind, since programming games was not an option when it was invented. However, the rules as initially described above can be programmed, and that's what I have done in Game Courier. The only issue with them is that they need emendation for a Pawn that moves twice on the same turn. If we keep those rules, then en passant capture should be impossible in this instance, or it should be allowed for the Pawn on its new space. I have the former programmed right now, whereas the latter would be trickier to program.