Chess Olympiads are chess events open to all national teams. They have been run since 1927 by the world chess organisation, FIDE. Since 1950, they have been run every two years. Earlier events were all-play-all. Then, as numbers grew, a system of qualifying sections were used. In recent times, the format has been a Swiss system. This, in chess terms, is a method whereby opponents are chosen each round to have similar scores. It has been widely used for individual tournaments.