Betting middles: two lines and a winning window

A line sets the boundary. Over 21.5 games wins from 22 games; under wins up to 21. A handicap adds or subtracts a specified amount from a team's score.

One sportsbook offers 21.5 games, another 23.5. Back over 21.5 and under 23.5: every result wins at least one leg. At 22 or 23 games, both win. That's the window.

Illustration: over 21.5 at 1.90, under 23.5 at 1.95, $100 on each side, $200 total.

Stakes needn't be equal. The middle calculator balances outside losses and shows the inside return. These amounts appear on the middles board.

A middle has positive expected value when its window hits more often than the break-even hit rate, where inside gains cover outside losses.

In this example, a rough average outside loss of $7.50 and an inside win of $185 imply a hit rate of about 4% to break even. The exact threshold depends on the outcome distribution. Assess these factors:

1. 01

   Loss outside the window

   A smaller outside loss lowers the threshold. Some pairs cost a fraction of a percent; pairs profitable outside the window are arbs too.

2. 02

   Window width

   All else equal, a wider window covers more results. Compare its width with the cost of missing it.

3. 03

   Window position

   A window near the most likely total generally has a higher hit probability than one at the extremes.

• Tennis: total games. Lines across sportsbooks can differ by a game or two.
• Basketball: points and handicaps. Many scoring increments allow windows covering several points.
• American football and baseball. Compare totals and handicaps across sportsbooks.
• Football. Windows often cover one score, such as exactly three goals.

An arb covers all outcomes. A middle usually has a small outside loss and a large inside win. It may produce a run of small losses before a window hits. Assess that downside before betting.