The fifty-move rule lets a player claim a draw if 50 moves by each side have passed with no capture and no pawn move.
Captures and pawn moves are the only things that permanently change the material and pawn structure, so they reset the counter. If neither happens for 50 full moves (that is, 50 by White and 50 by Black), the game can be drawn on a claim.
The rule exists to stop a player from shuffling pieces forever in a position where no progress is possible — for example, when the side with extra material cannot find a way to force checkmate. Without it, drawn endgames could run indefinitely.
In practice it most often appears in tricky endgames like rook-and-bishop versus rook, where the stronger side must convert within the move limit or settle for a draw.
Any capture or any pawn move resets it to zero. Because those are the only moves that irreversibly change the position, they restart the count toward a draw.
Over the board it is a claim — a player must claim the draw. Online it is usually enforced automatically. There is also a separate automatic draw at 75 moves under FIDE rules.
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