Online sportsbooks available in most U.S.Betting odds represent favorable outcomes in a game.
If it's a plus, you'll win more than $100 on a $100 bet and a minus means you'll have to bet more than $100 to win $100. They are in 100 because it's a 1:1 ratio, which means for every $1 you bet, you'll win $1 if your bet wins.
