Match & Game Format
Every badminton match is set up in a best-of-three-games format. Each game is played to 21 points with a required margin of victory of two or more points. If a team reaches 21 points with only a one-point advantage, then the game continues until one team either gains a two-point lead or reaches 30 points.
After each game, teams switch sides with their opponents. If a match should go to a third game, teams change sides midway through the game when the first team reaches 11 points.
Read more at: http://badminton.isport.com/badminton-guides/rules-etiquette-for-competitive-badminton