In sports betting there are a wide variety of wagers that can occur, ranging from taper bets to parlays to fixed-odd bets. The latter are a type of bet made when an individual wagers against the odds offered by a bookmaker or on a bet exchange system at one of the various betting sites on the internet. It is customary when using this system to actually know the odds at the time you place a wager. Most bookmakers will price up a book in order to make sure the net outcome is always in their favour, or to make sure the sum of the probabilities quoted for all possible outcomes will be in excess of 100%, therefore guaranteeing the bookmaker a profit. However, it is absolutely impossible for a bookmaker to know the absolute probabilities for any competition due to nature of human influence, so there is always some measure of odds regardless of how fixed one might think they are.
Sports betting has evolved over the years, and with the advent of the internet and betting sites has come the natural evolution of the system, with betting sites operating in many ways like a stock exchange, allowing odds to be set in the course of trading between individuals, with fractional odds being the favourite of bookmakers in the UK and Ireland. Fractional odds quote the net total paid out to the bettor should they win against the fixed-odds in sports betting, allowing for more accurate predictions by the bookmakers themselves.