Byes
By looking at a single-elimination bracket it quickly becomes clear that tournaments are easy to run only if they have a number of players which is a power of 2: i.e. 8, 16, 32, 64, 128, 256, etc. Having this exact number of players ensures that all players have somebody to compete against every round. In tournaments open to the general public, it is unlikely that this exact number of players will enroll, and in any case players may decide to voluntarily drop from the tournament at any point, so "byes" are used to fill holes in the bracket.
A player gets a bye when there is simply no opponent for him to play that round. Getting a bye is considered fortunate for a player; he is guaranteed to advance to the next round. Sometimes many byes are granted in a particular round because of holes in the bracket. The aim of the byes is to have the number of players be equal to a power of 2 in the next round, or possibly the round after that.
In some tournaments such as the playoffs of the NFL, byes are prizes to be earned by teams who do well in previous play.
Tiebreakers
Sometimes a "tiebreaker" statistic is needed to separate players who have the same win-loss record, particularly for the purpose of awarding prizes to the top players. For example, after five rounds of play in a Swiss-style tournament, 4th through 7th places are often taken by players who all have a 3-2 record. Often-used tiebreakers are score averages in the individual games played so far in the tournament, opponents' winning percentages, the total number of points scored by the player in the tournament, the total number of points scored by all the player's opponents in the tournament, and so forth. (The inability to boost one's tiebreaker statistics is considered the only disadvantage of receiving a bye in a previous round.)
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