The simplest analogy for explaining margins is betting on a coin toss. When playing with a friend, you might bet £10 on heads or tails. Under these terms, the odds will be 2.0, which reflects the actual probability (50 percent) of the event occurring. A bookmaker will add the margin, giving 1.91 odds for the two equally probable outcomes.
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Without the margin, the player will have to win the half not to lose any money. However, using the coin toss as an example, you would need to win some 53 percent of bets (at 1.91 odds) to break even. Thus, a bookmaker gains an advantage.
Bookmakers control the odds in such a way that they would win, thanks to margin, regardless of the outcome.
The higher the margin, the lower are the odds and chances to outpace a bookmaker in the long run. Generally, the lower the margin, the narrower is the action line. The lowest-margin bookmakers are the ones for professional punters.