Double elimination is a format for athletic tournaments in which a group of teams will compete against each other according to a preset structure, being eliminated from play after two losses. When only one team remains with less than two losses, they are declared the champions. The format is frequently used in amateur and collegiate baseball and softball leagues to determine season champions.
The double elimination format can be used for any number of teams, but is most suitable for four or eight team competitions. When the number of teams is not a power of two, the tournament format must be set up so that some teams will be given a bye at some point to make the number of matches work out. When more than eight teams are involved, the number of matches can becomes too large to complete in a reasonable time.
In setting up the tournament format, the teams are first seeded, or ranked. This is usually based on a comparison of team records during the prior season. The first round of competition will match the top seed against the lowest seed, the second seed against the second lowest, etc. If an odd number of teams is competing, the top seed may be given a bye in the first round. The winners of the first round games will play each other in the winner's bracket and the losers of the first round games will play each other in the loser's bracket in the second round. Losers in the loser' bracket are eliminated, having lost two games.
This is a good example of the bracket diagrams for a four team double elimination tournament. It shows the brackets for all sixteen of the Regional tournaments for the 2009 NCAA Baseball Championship Tournament.
The US NCAA Division I Baseball Championship is determined by setting 64 teams in sixteen Regional double elimination tournaments of four teams each. The sixteen Regional winners are paired into eight Super-Regional tournaments in which two teams play a best-of-three game set, which is simply a two team double elimination tournament. The eight Super-Regional winners meet in the College World Series in Omaha, Nebraska.
In the College World Series, the eight teams are divided into two brackets which each play a four team double elimination tournament, with the two bracket winners meeting for a best of three (or two team double elimination) contest, in which the winner is declared the National Champion.