A player may claim a draw after 50 moves-does this mean 25 moves by each player, which results 50, or 50 moves by each player, which results 100?
2 Answers
From the FIDE rules (emphasis added):
9.3
The game is drawn, upon a correct claim by the player having the move, if:
a. he writes his move on his scoresheet and declares to the arbiter his intention to make this move, which shall result in the last 50 moves having been made by each player without the movement of any pawn and without any capture, or
b. the last 50 consecutive moves have been made by each player without the movement of any pawn and without any capture.
So it's 100 total plies, or "half-moves."
An interesting detail of the 50 move rule: As only the player to move may claim a draw (either by declaring that the 50 move limit has been reached or by declaring his intention to make a move after which the limit will be reached), if the 100th half-move that is not a capture nor a pawn move is a checkmate, then the game is not drawn. See e.g. Arbiter's notebook 98.

- 3,390
- 1
- 18
- 30
30. h5
there have been no pawn moves until80. ... a4
, or until80 ... Ra8++
, or until80 ... Ra7+ 81. Kg8 Ra8++
. Would the game be deemed to have ended after White's eightieth move, or for White to claim the draw would he have to summon the director prior to making his eightieth move? Is a player allowed any means of determining at which move the draw will occur without the jeopardy of summoning the director at what he hopes is the proper time? – supercat Oct 10 '14 at 19:19