In a soccer match, for example, the real
probabilities of each of the 3 possible outcomes (home-win, draw and
away-win), based on their relative abilities may be 50%, 30% and 20%.

The total of these three percentages is 100%, thus representing a fair
'book'.

The real 1X2 odds of winning for each of the three possible outcomes
are 2.00 , 3.33 and 5.00 respectively.

In order to generate a profit on
the wagers accepted by the bookmaker he may decide to increase the values
to 53%, 33% and 23% for the three , representing 1X2 odds against of
1.89 - 3.03 and 4.34

These values now total 109%, meaning that the book has an
overround of 9% (109 − 100).

This value of 9 represents the amount of profit for the bookmaker if
he accepts bets in the correct proportions on each of the 3 possible
outcomes.

The art of bookmaking is that he will take in, for example,
$109 in wagers and only pay $100 back (including stakes) no matter which
team wins or a draw.

This is a low overround for 1X2 betting.