The first paper on the subject by Claude Shannon, published in 1950 before anyone had programmed a computer to play chess, successfully predicted the two main possible search strategies which would be used, which he labeled 'Type A' and 'Type B'.

Type A programs would use a "brute force search" approach, examining every possible position for a fixed number of moves using the minimax algorithm. Shannon believed this would be impractical for two reasons.

Firstly, with approximately 30 moves possible in a typical real-life position, he expected that searching the 30 × 30 × 30 × 30 × 30 × 30 positions involved in looking three moves ahead for both side (six plies) would take about 16 minutes, even in the "very optimistic" case that the program evaluated a million positions every second. (In the event, it took about forty years to achieve this speed.)

Secondly, it ignored the problem of quiescence, trying to only evaluate a position that is at the end of an exchange of pieces or other important sequence of moves ('lines'). He expected that adapting type A to cope with this would greatly increase the number of positions needing to be looked at and slow the program down still further.

Instead of wasting processing power examining bad or trivial moves, Shannon suggested that type B programs would use a "strategic AI" approach to solve this problem by only looking at a few good moves for each position. This would enable them to look further ahead ('deeper') at the most significant lines in a reasonable time.

Adriaan de Groot interviewed a number of chess players of varying strengths, and concluded that both masters and beginners look at around forty to fifty positions before deciding which move to play. What makes the former much better players is that they use pattern recognition skills built from experience. This enables them to examine some lines in much greater depth than others by simply not considering moves they can assume to be poor.

More evidence for this being the case is the way that good human players find it much easier to recall positions from genuine chess games, breaking them down into a small number of recognisable sub-positions, than completely random arrangements of the same pieces. In contrast, poor players have the same level of recall for both.

The problem with type B is that it relies on the program being able to decide which moves are good enough to be worthy of consideration ('plausible') in any given position and this proved to be a much harder problem to solve than speeding up type A searches with superior hardware.

One milestone was the abandonment of type B in 1973 by the team from Northwestern University responsible for the 'Chess' series of programs, who had won the first three ACM Computer Chess Championships (1970-1972). The resulting type A program, Chess 4.0, won that year's championship and its successors went on to come second in both the 1974 ACM Championship and that year's inaugural World Computer Chess Championship, before winning the ACM Championship again in 1975, 1976 and 1977.

One reason they gave for the switch was that they found it less stressful during competition, because it was difficult to anticipate which moves their type B programs would play and why! They also reported that type A was much easier to debug in the four months they had available and turned out to be just as fast: in the time it used to take to decide which moves were worthy of being searched, it was possible to just search the lot.

Temporarily, the brute force camp won out for these reasons and in the sense that their programs simply played better chess. The game of chess is not conducive to inerrantly discriminating between obviously bad, trivial, and good moves using a rigid set of rules. Traps are set and sprung by expert players who understand and master the many levels of depth and irony inherent to the game.

Furthermore, technological advances by orders of magnitude in processing power have made the brute force approach far more incisive in the 90's than was the case in the early years. The result is that a very solid, tactical AI player has been built which is errorless to the limits of its search depth and time. It turned out to produce excellent results, at least in the field of chess, to let computers do what they do best (calculate) rather than coax them into imitating human thought processes and knowledge. In 1997 Deep Blue, a Type A machine, defeated World Champion Garry Kasparov, marking the first time a computer has defeated a reigning world chess champion in standard time control.

However, starting in the late 1990s Type B programs began replacing Type A as the preferred method for programmers. In 1998, Rebel 10, a commercially available Type B program, defeated Viswanathan Anand who at the time was ranked second in the world, by a score of 5-3. But one should note that out of the four blitz games (5 minutes + 5 seconds Fischer for each move), Rebel won 3-1, in the two semi-blitz games (15 minutes) Rebel won 1.5 -0.5 and in the tournament game (40/2:00, 1 hour), it was Anand who won. Noteworthy is that computers play better at faster time controls, but a player's actual strength is only determined at the classical time controls, where Anand proved that Humans were still better*. In the early 2000's, commercially available Type B programs such as Deep Junior and Fritz were able to draw matches against former world champion Garry Kasparov and former "classical" world champion Vladimir Kramnik. In 2005, Hydra, a Type B chess machine, defeated Michael Adams who at the time was ranked 7th in the world, by an overwhelming score of 5.5-0.5.

On the other hand, it remained an open question whether any amount of brute force computation would ever be adequate to defeat the expertise of top humans. In 1968, International Master David Levy made a famous bet that no chess computer would be able to beat him within ten years. He won his bet in 1978 by beating Chess 4.7 (the strongest computer at the time), but acknowledged then that it would not be long before he would be surpassed. In 1989, Levy was crushed by the computer Deep Thought in an exhibition match.


Deep Thought, however, was still considerably below World Championship Level, as the then reigning world champion Garry Kasparov demonstrated in two sterling wins in 1989. It wasn't until a 1996 match with IBM's Deep Blue that Kasparov lost his first game to a computer at tournament time controls in Deep Blue - Kasparov, 1996, Game 1. This game was, in fact, the first time a reigning world champion had lost to a computer using regular time controls. However, Kasparov regrouped to win three and draw two of the remaining five games of the match, for a convincing victory.

In May 1997, an updated version of Deep Blue defeated Kasparov 3.5-2.5 in a return match. IBM keeps a web site of the event. While not an official world championship, the outcome of the match is often taken to mean that the strongest player in the world is a computer. Such a claim is open to strong debate, as a truly fair human-machine match is difficult to arrange.


IBM retired Deep Blue after the match and it has not played since. However, other "Man vs. Machine" matches continue to be played. In October 2002, Vladimir Kramnik and Deep Fritz competed in the eight-game Brains in Bahrain match, which ended in a draw. Kramnik won games 2 and 3 by "conventional" anti-computer tactics—play conservatively for a long-term advantage the computer is not able to see in its game tree search. Fritz, however, won game 5 after a severe blunder by Kramnik. Game 6 was described by the tournament commentators as "spectacular." Kramnik, in a better position in the early middlegame, tried a piece sacrifice to achieve a strong tactical attack, a strategy known to be highly risky against computers who are at their strongest defending such attacks. True to form, Fritz found a watertight defence and Kramnik's attack petered out leaving him in a bad position. Kramnik resigned the game, believing the position lost. However, post-game human and computer analysis has shown that the Fritz program was unlikely to have been able to force a win and Kramnik effectively sacrificed a drawn position. The final two games were draws. Given the circumstances, most commentators still rate Kramnik the stronger player in the match.

Hydra, a dedicated chess computer with custom hardware and sixty-four processors, has defeated other top computer programs in recent matches, and is probably the strongest computer player yet created. It crushed seventh-ranked Michael Adams 5.5-0.5 in a recent six-game match (though Adams' preparation was far less thorough than Kramnik's 2002 series). Some commentators * believe that Hydra will ultimately prove clearly superior to the very best human players, or if not its direct successor will.

Several other matches with computers against top human players were played recently:

In January 2003, Kasparov played Deep Junior, another chess computer program, in New York. The match ended 3-3.

In November 2003, Kasparov played X3D Fritz. The match ended 2-2.