Originally posted by tvochess
Captures and pawn moves are the only irreversible chess moves. A draw can be claimed after 50 half-moves without capture or pawn moves.
Correction: captures, pawn moves and castling
or moving a previously unmoved king or rook
are the only irreversible chess moves. In theory, therefore, castling
should also be mentioned in the 50 move rule; but who potters about for 50 moves when he hasn't castled yet?
Oh, and another correction: that's 50 full moves, not 50 half-moves. FIDE rule 5.2e.