Margin

Convert every price on a three-way football market into the chance it implies and add them up. A fair book totals one hundred per cent. A real one totals more, and the excess is the margin: it is the book's cut, taken from every stake on that market whether it wins or loses.

A two per cent book and a nine per cent book are not two opinions about a match, they are two different products, and the difference compounds across a season far more reliably than any welcome offer. The trouble is that one book does not have one margin. The same operator can price the top European leagues at two per cent and a friendly at eleven, so a figure is meaningless without the sample it came from.

That is why the margin thread here splits its figures into the ones computed across a whole football book and the ones computed on a handful of fixtures, and refuses to sort the two together. A top-flight-only figure is the flattering end of any book's range, and printing it beside an all-football median would make the wrong book look sharper.