3d move

[flok]

Hi,

Maybe a strange question but what thing can happen for the first time in a game?
E.g. castling or en-passant or whatever.

[mkchan]

Hi,

Maybe a strange question but what thing can happen for the first time in a game?
E.g. castling or en-passant or whatever.

Do you mean "What can happen _only once_ in a game"?
If so, I believe only castling(once per side), checkmate and stalemate can happen once in a game.
A double pawn move can happen once for each pawn(as long it's on the 2nd rank for white or 7th for black)
Enpassant can therefore happen only once for each pawn(immediately after the double pawn push)