Spread betting is a form of gambling on the outcome of any event where the more accurate the gamble, the more is won and conversely the less accurate the more is lost. A bet is made against a 'spread' (or index), on whether the outcome will be above or below the spread. The amount won or lost depends on the level of the index at the end of the event. The spread represents the index firms margin.

The concept has a long history in American sports betting and was imported to the United Kingdom in the 1980s. In North America the bettor usually bets that the difference in the scores of two teams will be less than or greater than a value specified by the bookmaker. For example, if a bettor places a bet on an underdog in an American football game when the spread is 3.5 points, he is said to take the points; he will win his bet if the underdog's score plus 3.5 points is greater than the favourite's score. If he had taken the favourite, he would have been giving the points and would win if the favourite's score less 3.5 points was greater than the underdog's score.

Spreads may be specified in half-point fractions to avoid ties, or pushes. The winner of a North American spread bet is paid off at even money, while a loss costs the bettor the amount wagered plus the bookmaker's commission, commonly known as vigorish or vig, usually 10 per cent; in the United Kingdom both sides are held at odds of 9-10. In North American betting a push is treated as if no bet at all had been made, while in the United Kingdom "dead heat" rules apply, resulting in a net loss of £5 on a £100 wager due to the 9-10 odds of the proposition.

If a key player on a side is marginally injured and may or may not play, the "sports book" - or establishment that handles the bets - may declare the game off-limits to bettors (by not quoting any spread at all on it), or may "circle" the game; in the latter scenario, lower maximum amounts for each bet are enforced (typically $5,000 instead of the $25,000 limit observed at most Las Vegas sports books) and certain specialty wagers, such as "teasers," are banned on either side in the game. (A "teaser" is a bet that alters the spread in the bettor's favour by a predetermined margin, often six points - for example, if the line is 3.5 points and the bettor wants to place a "teaser" bet on the underdog, he takes 9.5 points instead; a teaser bet on the favourite would mean that the bettor takes 2.5 points instead of having to give the 3.5. In return for the additional points, the payout if the bettor wins is less than even money. At some establishments, the "reverse teaser" also exists, which alters the spread against the bettor, who gets paid off at more than even money if the bet wins).

In the United Kingdom spread betting has come to resemble the futures market. The bets are usually on the outcome of sporting events or indeed on financial instruments (check out this site for more information on spread betting (http://www.financial-spread-betting.com)), but the firms often offer bets on more arbitrary events - such as the number of corners during a football match or the total shirt numbers of the goal scorers.

Unlike fixed odds betting the amount won or lost can be very large, as there is no single stake to limit the maximum losses. However, it is usually possible to place a "stop loss" with the bookmaker, automatically closing your bet if the value of the spread moves against you by a specified amount. "Stop wins" are the opposite -- closing your bet when the spread moves in your favour by a specified amount.

Example: In a football match between Liverpool and Everton the spread for corners is 12-13, the index firm believes there will be 12 or 13 corners in total during the match. A bettor approaches the firm with the belief that there will be more than 13 corners during the game, the bettor 'buys' at £25 a point at 13. If the final total of corners is 16 the bettor has won, receiving 3 x £25. If the final total of corners is 10, the bettor loses 3 x £25. A 'sell' transaction is similar except made against the bottom value of the spread. Often there is live pricing, which changes the spread during the course of an event allowing a profit to be increased or a loss minimized.

In North American sports betting many of these wagers would be classified as over-under (or, more commonly today, total) bets rather than spread bets.

Many spread firms will close your account if you win regularly.

See also Bet exchange, Bookmaker, Parimutuel gambling and point shaving.