[Retros] En-passant chess

peter.fayers at virgin.net peter.fayers at virgin.net
Sun Mar 25 12:20:28 EDT 2007

I thnik the paradox can be resolved if we consider the rationale behind the orthodox e.p capture,
1 Pd2-d4 Pe4xd3 e.p.
The assumption is that, as soon as White begins moving his Pawn, Black can start his response, in effect making an (almost) simultaneous move. It captures the Pawn on d3 before it ever reaches d4.

>From this, it follows that a unit captured en passant did not reach its destination square, so any unit it apparently captured has to be replaced.

Now the paradox. Consider White Ra5, Bf8, Black Bh4.
Play 1 Bf8-c5 Bh4xBe7ep; 2 Ra5xBg5ep.
Having captured the Black B on g5, it never reached e7, so couldn't have captured the wB. So replace this where it originally landed on c5. But this blocks the path of the wR a5 to g5, so ......
Go back to basics, Black is entitled to start moving Bh4 to e7 as soon as White start moving Bf8, but White cannot start moving Ra5 as Bh4 starts, because HE IS STILL MOVING HIS BISHOP ! Whereas we can accept a player starting his move whilst his opponent is still moving, we cannot allow a player to start a second move (Ra5 - b5 - c5 ..) while still making his previous move Bf8-e7.

>From this we get the logical rule that there may not be two successive en passant captures. Or, "A player may not make an en passant capture of a unit that has just made an ep capture of its own".

This rule resolves the paradox.
Regarding Knight moves, N moving b1-c3 leaps over any units, enemy or friendy, on b2 or c2, so what difference does an enemy unit landing there mid-move make? None. So rule is simple: Knights cannot be captured ep.
Moer complex is the line-of-action capture.
Bf2-c5 : can it be captured en passant by either Qa7-e3 or Qg1-e3? My instinct says no, the ep capture should come in from the side. However, the "simultaneous move" rationale I applied above suggests that both those captures should be allowed. I'm open on this one.

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