[1] Following his work on the Unix operating system, Ken Thompson turned his attention to computer chess.
In competition, this early version encouraged Thompson to pursue a brute-force approach when designing Belle's hardware.
The initial chess program was rewritten to utilize move-vs-evaluation quiescence search and evaluate positions by prioritizing material advantage.
[3] In 1976, Joe Condon implemented a hardware move generator to be used with software version of Belle on the PDP-11.
Over the next year, Belle played several UCSF games and finished 3-1 in the 1974 ACM Computer Chess Championship.
[9] In 1980, the third generation of Belle won the third World Computer Chess Championship in Linz, Austria.
[12] In 1982, the United States Customs Service impounded Belle for violating the Export Control Act as Thompson attempted to travel with it to the Soviet Union for a chess exhibition.
Thompson said that the computer was made of commercial off-the-shelf components, and that its only military use was "to drop it out of an airplane.
Belle's reign ended when it placed sixth in the Fourth World Computer Chess Championship, despite being the favorite to win.
Thompson concluded that for each additional level of search, Belle improved by approximately 250 rating points.