Interzonal

Interzonal chess tournaments were tournaments organized by the World Chess Federation FIDE from the 1950s to the 1990s. They were a stage in the triennial World Chess Championship cycle and were held after the Zonal tournaments, and before the Candidates Tournament. Since 2005, the Chess World Cup has filled a similar role.

In the first year of the cycle, every FIDE member nation would hold a national championship, with the top players qualifying for the Zonal tournament. The world was divided into distinct zones, with the USSR, United States and Canada each being designated a zone, thus qualifiers from these three zones went directly to the Interzonal. Smaller countries would be grouped into a zone with many countries. For example, all of South America and Central America combined originally formed one zone.

The top players in each Zonal tournament would meet in the Interzonal tournament, which would typically have between 20 and 24 players. The top finishers (e.g., the top six in 1958) would qualify for the Candidates Tournament, which would take place the following year. They would be joined in the Candidates by the loser of the previous candidates' final and the loser of the previous world championship match. The winner of the Candidates tournament would play a 24-game match with the World Champion the following year, and need to win outright to gain the title.

To illustrate, the 1957 US Championship served as a Zonal tournament. The top three, Bobby Fischer, Samuel Reshevsky, and James Sherwin qualified for the Interzonal tournament that was held in Portorož in 1958. The top six at Portorož, (Mikhail Tal, Svetozar Gligorić, Tigran Petrosian, Pál Benkő, Friðrik Ólafsson and Fischer) qualified for the Candidates tournament. They were joined by Vasily Smyslov (loser of the 1958 rematch for the championship with Mikhail Botvinnik), and Paul Keres, who had finished second at the 1956 Amsterdam Candidates tournament. Those eight played a quadruple round-robin in the 1959 Candidates tournament. The winner was Tal, who then played Botvinnik a match for the championship in 1960.

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