Fifty-move rule
The fifty-move rule in chess states that a player can claim a draw if no has been made and no pawn has been moved in the last fifty moves (for this purpose a "move" consists of a player completing a turn followed by the opponent completing a turn). The purpose of this rule is to prevent a player with no chance of winning from obstinately continuing to play indefinitely or seeking to win by tiring the opponent. Chess positions with only a very few pieces can be "solved", that is, the outcome of best play for  both sides can be determined by exhaustive analysis; and if the outcome is a win for one side or the other (rather than a draw), it is of interest to know whether the defending side can hold out long enough to invoke the fifty-move rule. The simplest common endings, called the basic checkmates, such as king and queen versus king, can all be won in well under 50 moves. However, in the 20th century it was discovered that certain endgame positions are winnable but require more than 50 moves (without a capture or a pawn move). The rule was  therefore changed to allow certain exceptions in which 100 moves were allowed with particular material combinations. However, winnable positions that required even more moves were later discovered, and in 1992, FIDE abolished all such exceptions and reinstated the strict 50-move rule. Statement of rule. The relevant part of the FIDE laws of chess is quoted below: A claim does not have to be made at the first opportunity; it can be made any time when there have been no captures or pawn moves in the last fifty moves. A game is not automatically declared a draw under the fifty -move rule; the draw must be claimed by the player whose turn it is to move. Therefore, a game can continue beyond a point where a draw could be claimed under the rule. When a draw under the fifty-move rule can be claimed, one of the players is usually happy to claim it. Games drawn under the fifty-move rule before the endgame are rare. One example was the game Filipowicz versus Smederevac, Polanica Zdrój 1966, where no captures had been made in the whole game. The game was drawn after move 70 by Smederevac on draw request by  Filipowicz, the last pawn having been moved on move 20 by Smederevac. Seventy-five-move rule. If seventy-five moves are made without a pawn move or capture being made, the game is drawn unless the seventy-fifth move delivers a checkmate. No claim needs to be made by either player, as the draw is mandatorily applied by the arbiter. Examples. Timman vs. Lutz, 1995. In this 1995 game between Jan Timman and Christopher Lutz, an endgame with a rook and bishop versus a rook occurred. White is striving for the winning Philidor position while Black is employing the drawing  Cochrane Defense and the "second-rank defense" (see rook and bishop versus rook endgame). Black was defending well in the difficult defense and could have claimed a draw on the 119th move. Lutz notes that he claimed a draw on move 121, ironically making a move that enters what would have been a losing position. Karpov vs. Kasparov, 1991. A draw by the fifty-move rule could have been claimed after Black's 112th move in a 1991 game between Anatoly Karpov and Garry Kasparov, but neither player claimed it. The last capture occurred on White's 63rd move (and  the last pawn move occurred before that). White could have written his 113th move (which wouldn't have been a capture or pawn move) on his scoresheet and claimed a draw. After White's 113th move, either player could have claimed a draw on his turn to move, without having to write down his next move. Instead, the game continued for several more moves: The players agreed to a draw at this point because after 115.Kxf6 the position is a stalemate. If 115.Ke8, then 115...Rxf5 116.Nxf5, and the position is clearly drawn because the two knights cannot force  checkmate (see two knights endgame). Lputian vs. Haroutjunian, 2001. An unusual event occurred in a game in the 2001 Armenian Championship between Smbat Lputian (who won the championship) and Gevorg Haroutjunian. The last pawn move was on White's 86th move and no captures occurred after it. (The game was a theoretical draw from before here until Black's 141st move.) Black could have claimed a draw after White's 136th move (or at any of the subsequent moves). Instead, the game continued and Black resigned on his 142nd move after White had achieved a forced winning position even though  the right to claim a draw was still in effect. Nguyễn vs. Vachier-Lagrave, 2008. Another unusual event occurred in the Aeroflot Open of 2008 in a game between Nguyễn Ngọc Trường Sơn and Maxime Vachier-Lagrave. The last capture was on move 71, creating a rook and bishop versus rook endgame. The game was a theoretical draw until Black blundered on his 113th move. White managed to find the win, but could not secure checkmate before the fifty-move rule came into effect, and Black claimed a draw. History. The rule has a long history. The purpose of the  rule is to prevent someone from playing on indefinitely in a position that cannot be won. A precursor to chess, shatranj, had a seventy-move rule. The fifty-move rule was introduced into chess by Ruy López in his 1561 book. Pietro Carrera (15731647) thought that twenty-four moves was the right number but Bourdonnais (17951840) argued for sixty moves. By 1800, a claim under the rule could be made only in certain specified types of endgame, which varied from one set of rules to another. The move-count started when the request to implement the rule was made  (instead of going back to the last capture or pawn move) and a capture or a pawn move did not reset the count. The rules used at the 1883 London tournament reset the count if there was a capture or pawn move, but still started the count when the claim to apply the rule was made instead of going back to the last capture or pawn move. At one time, it was believed that all winnable endgames could be won within fifty moves. However, in the early 20th century, some exceptions were found, including A. A. Troitsky's (1866-1942 ) analysis of the two knights endgame as well as the endgame of a rook and bishop versus a rook. The rules of chess were revised several times to admit exceptions to the fifty-move rule for certain specific situations. Early on, the fifty-move rule applied to games but not to games. During the time periods when the fifty-move rule admitted exceptions, there were a number of revisions. In 1928 FIDE enacted rules that if an endgame theoretically requires more than 50 moves to force checkmate, twice that number of moves were allowed. For instance, in the rook and  bishop versus rook endgame, 132 moves were allowed, since it was twice the 66 moves that were thought to be required at that time. (The actual maximum number of moves needed is 59.) In 1952 FIDE revised the law, allowing for 100 moves in such positions but requiring that players agree to an extension for these positions before the first move is made. This was still in effect in 1960. The positions were not specified in the rules, to allow for the possibility of more positions requiring more than 50 moves to be discovered (which is what happened). The following  positions were understood to require more than 50 moves: Article 12.4 of the 1965 FIDE rules states: The number of moves can be increased for certain positions, provided that this increase in number and these positions have been clearly established before the commencement of the game. Harkness notes that "Some of these unusual positions have been established and accepted by FIDE", including two knights versus a pawn. The 1975 and 1977 versions of the rules included the same wording (not specifying the positions or the number of moves). In 1984, the rule was modified and it became Article 10.9. Now  100 moves were explicitly specified and the positions above were listed in the rule. (The wording about the positions and number of moves having to be specified in advance of the game was dropped.) Ken Thompson's investigations in the 1980s using the Belle chess computer discovered numerous endgames winnable in more than 50 moves. However, these often involved seemingly random moves that defied human comprehension or analysis, in situations that would hardly ever occur in real gameplay. In 1989 the rule (still Article 10.9) was changed to 75 moves, and the listed positions were: The rule was then changed  to allow just 50 moves in all positions. Some sources say that the 1989 rule was in effect for only a "year or so" or a "few years". but one source of the 1992 rules gives the pre-1984 wording: "...increased for certain positions if it was announced in advance". By 2001 the rule was Article 9.3 and allowed 50 moves for all positions. Research into how many moves are required to win certain endgames continued. Exhaustive retrograde analysis using faster computers to build endgame tablebases has uncovered many more such endgames, often of previously unsuspected length. In 2008, the record  was 517 moves (assuming optimal play by both sides) to make a piece capture or exchange that achieves a simpler and more obviously winnable sub-endgame, for a particular position involving a queen and knight versus a rook, bishop, and knight. In 2013, this record was improved to 545 moves. Many of the longest games on record involve the rook and bishop versus rook endgame, when the rule for more moves was in effect. (See pawnless chess endgame and rook and bishop versus rook endgame.)