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In theory, a betting book can be fairly weighted between bookmaker and punter. However, to ensure a profit margin, a bookmaker will alter the odds in their favour. Overround is a means of expressing to what extent the odds are in favour of the bookmaker. An evenly weighted book is expressed as 100%, and the more the odds move in the bookmaker’s favour the more that figure rises. Thus a book that is weighted 20% in favour of a bookmaker is expressed as 120% overround.