Interzonal
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Interzonal chess tournaments were tournaments organized by FIDE, the World Chess Federation. They were a stage in the World Chess Championship cycle. Basically, the plan was that the cycle would last three years. In the first year, every FIDE member nation would hold a national championship. The top players would qualify to the Zonal tournament. The world was divided into zones. Large countries such as the USSR and the United States had their own zone. Smaller countries would be grouped into a zone with many countries. For example, all of South and Central America combined was originally just one zone. There were originally no Asian or African zones, because there were no countries in those continents that were members of FIDE.
The top players in each Zonal tournament would come together and play in the Interzonal tournament. Typically, the Interzonal tournament would have about 24 players. Then, the top six would qualify to the Candidates Tournament, that would take place the following year. Those six would join with the top two from the previous candidates tournament from three years earlier. These eight players would play mini-matches against each other. The winner of those matches would play a 24 game match with the World Champion the following year.
To illustrate, in 1957 the US Championship was held. The top three, Bobby Fischer, Samuel Reshevsky, and James Sherwin qualified to the Interzonal Tournament that was held in Portorož in 1958. The top six at Portorož, which included Mikhail Tal, Svetozar Gligorić, Tigran Petrosian, Pál Benkő, Friðrik Ólafsson and Bobby Fischer qualified to the Candidates tournament. They were joined by Vasily Smyslov and Paul Keres who had been the top two at the 1956 Candidates tournament. Those eight played four games each against each other in 1959. The winner was Mikhail Tal, who then played Mikhail Botvinnik a match for the world chess championship in 1960. Also in 1960, a new three-year cycle, with Bobby Fischer, William Lombardy and Raymond Weinstein qualifying from the US Chess Championship, started.
Thus, Interzonal tournaments were held every three years from 1948 until 1993. However, by 1972, the system was becoming unwieldy. There were far too many top level players and the cost of staging these events had become too great. Also, new countries were joining, especially in Asia. China, India, Indonesia and the Philippines started producing top level grandmasters where there had been none before. Even Africa was demanding two zones. It was not practical to put all the top players in a round robin tournament. So in 1973 the system was changed to two Interzonals (with usually the first three in each qualifying for the Candidates), then in 1982 it changed again to three Interzonal tournaments. Further increases led to the Interzonal being staged as a single Swiss system tournament in 1990 and 1993.
The most famous Interzonal tournaments were Stockholm 1948 won by David Bronstein, Saltsjöbaden 1952 won by Alexander Kotov, Portorož 1958 won by Mikhail Tal, Stockholm 1962 won by Bobby Fischer and Sousse 1967 won by Bent Larsen. (Note: the 1962 Interzonal was supposed to have been held in 1961 under the three-year schedule, but had to be postponed one year because of lack of funds.)
The last FIDE Interzonal was Biel 1993, won by Boris Gelfand. The short-lived Professional Chess Association also held one Interzonal, in 1993.
